PURPOSE: To constantly keep a mowing edge in sharp state by providing an actuator driven by electric power to reciprocate a pair of mowing edges relative to each other and providing a controlling apparatus to control the reciprocation switching action and period of the actuator.

CONSTITUTION: A pair of mowing edges 43, 45 having dentiform edge parts 49, 51 at equal intervals are placed one upon another in a state to enable relative reciprocating motion. The mowing edges 43, 45 are relatively reciprocated by actuators 71, 71' driven by electric power or hydraulic pressure. The reciprocation switching action and switching period of the actuators 71, 71' are controlled by a controlling apparatus 87. As a result, a grass can be instantaneously cut to constantly keep the sharpness of the edge and the period of opened state of the edge can be prolonged to facilitate the intrusion of grass between the edges.