The present invention reduces weight while assuring required rigidity of a piston reduces cost of the piston. A piston includes first and second pin bosses supporting a piston pins, a first side wall coupling the first pin boss and a piston skirt, a second side wall coupling the second pin boss and the piston skirt, and a pair of bridges coupling the first and second side walls and forming upward spaces just above. First and second through holes are formed in the first and second side walls. The first and second through holes and the upper spaces are formed by a single punching process using a punch and, simultaneously, the bridges are formed.