We should divide this question into several pieces.
1. Did anybody in IJA infantry company receive an appointment on paper as a sniper?
No. In IJA system, even assignment of 1st and 2nd lieutenants to a company was not specified on paper. IJA attached them to a regiment (battalion or unit sometimes, anyway the echeron next to division) on paper. Regiment leader assigned him to a company. The rule on the army formation simply said 'at war a company shall be divided into platoons and squads'. Of cource even in peacetime they had formations and appointments in the platoon (say, LMG gunner) but it was in quite informal manner.
2. Did IJA use sniper rifles?
Yes.
http://en.wikipedia.org/wiki/Type_97_sniper_rifle
Type 99 rifle had similar variation with scope.
3. Did IJA officers give a soldier snipe mission?
Approximately yes.
Infantry drill manual in 1940 (歩兵操典) article 122 reads
Shooting shall be conducted at first by LMG, if necessary by LMG and snipers(狙撃手), if situation demands by snipers. Closing the enemy, if increase of firepowere is needed, add weapons as needed.
Note that when increasing the firepower placing too many rifles onwards often hinders frendly heavy weapons and incur fruitless loss before an assault.
Also article 123 says
Usually LMG shall shoot preferrable target among attack (shoot) targets of his squad. Riflemen shall choose relatively distinct target on confronting part. Sniper shall immediately shoot especially preferrable target, such as enemy leader, outpost, automatic weapons coming in sight near attack target. If the squad leader did not deploy LMG, he let sniper shoot preferrable target among attack (shoot) targets of his squad. When squad leader let sniper shoot anything out of attack targets, he choose objects dangerous to the troop or that need to be supressed inmmediately
Infantry drill manual in 1940 (in prewar literary Japanese)
http://www.warbirds.jp/sudo/infantry/souten_index.htm
I think 'sniper' in this article 123 refer to 'marksman' in modern infantry.
4. By the way I know I learned Japanese but I cannot read Infantry drill manual in 1940 at all.
No? Less and less Japanese can read them.
5. Did some soldiers always act as snipers? In other words, Sniper soldier was fixed?
Perhaps yes. Sharpshooter medal (射撃徽章) was given to excellent shooter (about 10% of soldiers) in annual competition. Even if there was no medal bearer in a squad, better shooter in each squad was identified. Perhaps they used scarce rifles with sniper scope.
Of course an isolated soldier might behave as sniper and in retreat some soldier might be ordered to remain as a sniper, as discussed in this thread.
6.How snipers worked in an IJA infantry squad?
I have a book 'Reference for Infantry Training vol.2 squad (歩兵教練の参考 第二巻 分隊)', Edited by army infantry school in 1940. In Ch.3 'Economizing and carrying of ammunition', a finely noted illustration. Summarizing the notes,
The authors (officers in the school) recommended the ammunition the leader and No.1 had 'should be used at last'. No.1 here is a squad subleader.
No.2-No.4 were LMG gunner, bullet feeder and bullet carrier. Japanese LMG used banana magazine only, so No.3 and No.4 were busy in refilling magazines in combat.
Some soldiers, illustrated as two, were snipers (狙撃手). Other squad riflemen shouls pass ammo to snipers if they were in short of ammo.
Comparing with infantry drill manual above, LMG and a few 'snipers' mainly provide the firepower of the squad.
p.s.
Later I hit upon an idea that some old posts here referred to books insisting that IJA troops used LMG as a sniper rifle. On article 122, first attack should be by LMG. If gunner spared bullet to minimum, it looked as if he behaved as a sniper.
