An infant restraint system for mounting on a vehicle seat having a seat portion and a back portion, the infant restraint system comprising: a back rest portion for disposition along the back portion of the vehicle seat and having a back rest surface which extends in a vertical direction; a seat portion of the infant restraint system being configured such that it is connected to a lower portion of the back rest portion swingably between an in-use position and a storage position, extends toward a front side of the back rest surface at the in-use position, and extends along the back rest surface behind the back rest surface at the storage position; upper-side position regulating portions for regulating upper-side positions of a pair of shoulder belts, the upper-side position regulating portions being formed on a left portion and a right portion of an upper portion of the back rest surface respectively; lower-side position regulating portions for regulating lower-side positions of the pair of shoulder belts, the lower-side position regulating portions being formed on a left portion and a right portion of the back rest surface at a position below the upper-side position regulating portions respectively or a left portion and a right portion of the seat portion respectively; an upper body belt portion which includes the pair of shoulder belts extending from the upper-side position regulating portions to the lower-side position regulating portions, and a pair of tongue plates which are respectively attached to the pair of shoulder belts in a state where the pair of shoulder belts are allowed to pass through the pair of tongue plates, respectively; and a crotch belt portion extending from a crotch belt mounting portion mounted on a center portion of a front edge portion of the seat portion of the infant restraint system, and having a buckle body which is detachably engaged with the pair of tongue plates on a distal end portion thereof, wherein a side guard portion extends frontward from a left edge portion and a right edge portion of a lower portion of the back rest portion. a back rest portion for disposition along the back portion of the vehicle seat and having a back rest surface which extends in a vertical direction; a seat portion of the infant restraint system being configured such that it is connected to a lower portion of the back rest portion swingably between an in-use position and a storage position, extends toward a front side of the back rest surface at the in-use position, and extends along the back rest surface behind the back rest surface at the storage position; upper-side position regulating portions for regulating upper-side positions of a pair of shoulder belts, the upper-side position regulating portions being formed on a left portion and a right portion of an upper portion of the back rest surface respectively; lower-side position regulating portions for regulating lower-side positions of the pair of shoulder belts, the lower-side position regulating portions being formed on a left portion and a right portion of the back rest surface at a position below the upper-side position regulating portions respectively or a left portion and a right portion of the seat portion respectively; an upper body belt portion which includes the pair of shoulder belts extending from the upper-side position regulating portions to the lower-side position regulating portions, and a pair of tongue plates which are respectively attached to the pair of shoulder belts in a state where the pair of shoulder belts are allowed to pass through the pair of tongue plates, respectively; and a crotch belt portion extending from a crotch belt mounting portion mounted on a center portion of a front edge portion of the seat portion of the infant restraint system, and having a buckle body which is detachably engaged with the pair of tongue plates on a distal end portion thereof, wherein a side guard portion extends frontward from a left edge portion and a right edge portion of a lower portion of the back rest portion.