To reduce the number of parts and adjusting work for a rear two- wheeler.

The rotationally moving shaft 35 of a lever 36 associated with a piston rod is axially mounted in a rotationally movable manner on a pair of brackets 34, 34 protruded on the body of a load sensing valve applied to the load of a rear two-wheeler to reduce braking pressure and the sensor shaft 41 of a start assisting device 10 for the rotationally movable shaft 35 is threadedly fitted thereto. A housing 43 to which a detector 46 is fixed is supported in a rotationally movable manner and coaxially with the sensor shaft 41 on a bearing part 42 formed on the bracket 34. The light emitting element 46a and the light receiving element 46b of the detector 46 are opposed to each other on the sensor shaft 41 with a detected element 44 fixed therebetween. The detector 46 is connected to the controller of the start assisting device with an electric wiring 47. In this way, the load sensing valve and the start assisting device are used to serve as the lever, so that the number of parts and lever adjusting work are reduced and the manufacturing cost of the rear two-wheeler is reduced.