A cap attaching structure includes a cylindrical member and a cap member. The cylindrical member has an end plate, a side wall to be inserted into the cylindrical member, and an engagement claw piece in which a proximal end portion is connected to an inserting side end portion of the side wall, while a distal end portion extends towards the end plate via a slit. On an outer circumference of the engagement claw piece, a claw portion adapted to be brought into engagement with an engagement hole of the cylindrical member is provided. The distal end portion of the engagement claw piece is disposed in a cutout portion in the end plate.