To provide an insert fastener for a helmet, which is easy to understand how to attach/detach an internal trim element to/from the helmet body, easy to conduct its attaching/detaching operation with a small force, and has such a structure that the insert does not come off inadvertently due to impact in attaching the insert.

The insert fastener for a helmet has the following structure: The inner surface near the rim of the helmet body has a pair of bracket parts formed from the rim side of the helmet body toward the depth side apart from each other with insertion grooves provided on the sides facing each other, an end part disposed between the pair of the brackets, and a base end fitted with an insert, and a pair of resilient fastening pawl members flexible inward which are formed projectedly on both left and right sides of the end part. This fastener also has insertion pieces each insertable from the rim side of the helmet body toward the depth side by bending the pair of resilient fastening pawl members into the insertion grooves, and engagement holes each provided in the bottom of the insertion groove and functioning to allow the insertion piece not to slip off by dropping the pawl part of the resilient fastening pawl member inserted into a predetermined position of the insertion groove.