The invention provides an operation unit of an engine provided with a throttle operating lever operating a throttle valve of the engine from an idle position to a full-open position, two terminals connected to an inner side of a drive circuit of a start motor, a contact element interposed between the terminals and setting a region between both the terminals to a conduction state or a non-conduction state, and a start switch making the region between the terminals conductive or non-conductive, the operation unit further comprising interlocking means for relatively moving a contact position between at least one terminal of the two terminals and the contact element in correspondence to an operation amount of the throttle operating lever, and the terminals and the contact element being brought into contact with each other only at a time when the throttle operating lever is at an idle position.