Abstract: The present invention relates to a reciprocating device for a power saw which includes a gear box, a main spindle, a rotary gear driven by the main spindle, a rotary transmission shaft fixed to the gear, a pair of support members and a reciprocating rod unit capable of moving reciprocatively, wherein the rotary transmission shaft and the main spindle are non-parallel to each other. The gear is arranged substantially between the pair of support members, thereby minimizing the space occupied by the internal structure of the reciprocating device and the weight of the power saw.