PURPOSE: To satisfy both requirements that safety of a head guard for a child is improved and oppression sense on the child is removed, in a seat device for a child such as a child's safety seat for an automobile which is provided with a pair of head guards projecting from both edge parts of a seat back in the front direction.

CONSTITUTION: Head guards 4, 5 are mounted on a seat back part 3 so as to rotate around the axis which is substantially extended parallel to the rising direction of the seat back 3. Therefore when a child is asleep, the distance between a pair of head guards is narrowed so as to provide the child with high safety and on the other hand, when the child is awake, time distance between the head guards is widened so as to remove oppression sense from the child.