An infant restraint system includes: a back rest portion; a seat portion connected to a lower portion of the back rest portion swingably between an in-use and storage position; upper side position regulating portions formed on a left and right portion of an upper portion of the back rest surface; lower side position regulating portions formed on a left portion and a right portion of the back rest surface at a position below the upper side position regulating portions; an upper body belt portion which includes the pair of shoulder belts and a pair of tongue plates attached to the pair of shoulder belts; a side guard portion respectively extending frontward from a left edge portion and a right edge portion of a lower portion of the back rest portion; and a crotch belt portion having a buckle body which is detachably engaged with the pair of tongue plates.