To assure safety by starting a machine body at a low speed.

An interlocking mechanism comprises an input case of which a base end part is interlocked with a mission part for free rotation around the base end part, a variable input pulley provided to the tip part of the input case, an output pulley attached to the output shaft of an engine, a transmission belt stretched between both pulleys, and an actuator for rotating the input case. The input case of the actuator is interlocked with switching operation with a forward/backward selecting lever for rotation, to adjust tension of a transmission belt, thus the variable input pulley provided to the input case is operated for changing speeds. A clutch mechanism is provided with a dog clutch. At the time of connecting the dog clutch, the input pulley rotates slower than a prescribed rotational speed, and the input pulley accelerates to a prescribed rotational speed after the dog clutch is connected.