A resilient returning device for a power operated piston tool comprises a one piece elastic bellow-shaped body including a plurality of repeating pleats, each of which has two pleat halves that cooperatively define a first elbow therebetween. Each of the pleat halves of each of the pleats cooperates with an adjacent one of the pleat halves of an adjacent one of the pleats to define a second elbow therebetween. Each of the first and second elbows has a bulging side. The bellow-shaped body is formed with a plurality of loop-shaped grooves that are indented from the bulging sides of the first elbows of the pleats or the bulging sides of the second elbows of the pleats, respectively.