PURPOSE: To prevent an operating lever from being mistakenly put in reverse, by providing a stopper in such a manner that it can be optionally changed over between an acting state and a non-acting state by a single operating member.

CONSTITUTION: Guide openings are provided in the top plate 16a of an operating member mounting box, which serves as a lever guide, so that operating routes 19a, 19b for operating levers 15a, 15b extend straightly in the front-to-rear direction of the body of a machine and in parallel with each other. A stopper 23 for making impossible to shift the operating levers 15a, 15b from neutral position N to reverse position is mounted on a rotary shaft 24 so that the stopper can be turned relative to the box 16 at will. When a single operating member 25 which is mounted on the the portion of the shaft 24 protruded out of the box 16 and can be turned together with the shaft is put in a first operating position ON, the stopper 23 is set in action. When the operating member 25 is put in a second operating position OFF, the stopper 23 is set out of action.