When a driving piston starts moving upward toward its top dead center or starts moving downward, a second engaging portion engageable with a second rack of a driver is configured to receive a force. The second engaging portion of a second wheel is provided with high strength and wear resistance. Because of this configuration, a cost of production and maintenance replacement parts can be reduced, in comparison with a case where a total mechanism, including first engaging portions of a first wheel, is provided with high strength and wear resistance.