To make it possible to replace only a faulty portion or a defective part during repair or replacement by installing main parts such as a plunger, vibration sensor, heater function, etc., on a substrate of microcomputer in one united body in a rectangular outer box.

An outer box 1 of a circuit breaker has contact portions 2, 3 in a rectangular box which are able to turn on and off the power supply; power input is performed by a manual switch 5, earthquake is sensed by a vibration sensor 8 which is operated by a certain seismic intensity; and by this sensing, a plunger 4 pushes a power supply cut switch 6 and contact portions 2, 3 are automatically turned off. In addition to the output power supply for the plunger 4 and a DC stabilizer, main parts such as heater function should be attached to a board 7 of a microcomputer in one united body. By doing this, only a defective part or a part to be replaced can be removed and replaced so that the repair work can be performed quickly at a lower cost to the user.