An operation unit is provided with an engine operation box storing a control operation portion controlling an opening degree of a throttle valve and a start switch terminal connected to a start motor; a throttle operating lever for controlling the opening degree from an idle position to a full-open position; and a start switch connecting and disconnecting the start switch terminal, wherein the throttle operating lever has a switch shielding portion which enables to operate the start switch without an interference between the throttle operating lever and an operating portion of the start switch when the throttle operating lever is at an idle position, and disables to operate the start switch by shielding at least a part of the operating portion of the start switch by means of a part of the throttle operating lever when the throttle operating lever is operated to be apart from the idle position.