To deny vibration caused by rectilinear motion so as to off set vibrations caused by a balance weight each other by using two nearly equal balance weights which are rotated in a reverse direction each other with the same rotational speed as rotational speed of an eccentric shaft.

A gear 20 fixed to the output shaft 1a of a motor 1 is provided with an eccentric weight part (a balance weight) 20a. A gear having the same numbers as the gear 20 is engaged with the gear 20, an eccentric weight part 21a is arranged on the gear 21, it is fixed to a shaft 22. The shaft 22 is supported by bearings 25, 26 arranged on a motor installing plate 23 and a bearing plate 24 connected to the installing plate 23 through a plurality of struts 27. The eccentric weight parts 20a, 20b are eccentrically in a direction opposite to the eccentric direction of the eccentric shaft 2. It is thus possible to prevent generation of vibration of operation direction of a reciprocating motion shaft such as a connecting rod 4 and generation of vibration of rectangular direction.