To provide an inexpensive impact sensor which can secure the stationary time of a moving member for long times and stablyto be the closing location of a contact part and furthermore improve reliability with a simple structure.

This impact sensor is formed by a casing 7 in which are housed similarly to its predecessors, a leas switch 1 which contains a contact part, opened or closed by the presence or absence of a magnetic force, a tube 2, a permanent magnet 3, and a spring 4 whose one end is brought into contact with the one side of the permanent magnet 3 and energizes the permanent magnet 3 to be movable along the outer surface of the tube 2 at the time of receiving a predetermined acceleration a. In this sensor, the section to be an closing location of a contact by a magnetic force at the tube 2 forms a double- level part. This double-level part contains a smooth double-level part DM provided at a part close to the side of the permanent magnet 3 in the stationary state of less than the predetermined acceleration a and a wall surface double- level part DS forming the wall surface local part of a front stopper 3 and matching to the wall surface of the front stopper 6 for colliding and stopping the movement of the permanent magnet 3.