To solve the following problem of a seat switch having a conventional configuration: on-off operations by a vertical sliding motion cause incorrect operation and the absence of sensitive adjustment function causes the switch not to be adjusted to proper sensitivity.

Between a seat plate 8 of a driver seat 7 and a lower seat bracket 9, there is provided a seat switch mechanism 15 which is operated downward by a driver's being seated to turn on the switch for engine start. Moreover, the seat switch mechanism 15 is provided at a predetermined angle θ to the vertical perpendicular direction. On the seat plate 8 side, there is provided sensitivity adjustment means 18 of adjusting the switch part sensitivity of the seat switch mechanism 15.