To provide an immobilizer capable of furnishing an inclination sensor capable of precisely detecting inclination of a vehicle in addition to controlling to make it possible or impossible to start an engine by judging whether an ID code is normal or not.

This immobilizer has a transponder 1 for a driver to carry and to transmit the intrinsic ID code of the vehicle, an antenna 2 to receive the transmitted ID code and a microcomputer 4 to judge whether the received ID code is a previously set normal one or not, to make it possible to start the engine when it is judged as the normal one and to make it impossible to start the engine when it is judged as not the normal one. A storage case C to store the antenna 2 is arranged in the neighborhood of a revolution axis Ha of a handle bar H with which the vehicle is furnished, and an acceleration sensor 8 as the inclination sensor capable of detecting the inclination of the vehicle is arranged in the storage case C.