PURPOSE: To provide a self-diagnosing device for semiconductor acceleration sensors which can easily and surely make discrimination.

CONSTITUTION: The output (lamp voltage) of an integrator 23 is impressed upon the mobile electrode 12 of an acceleration sensor 15. A discrimination circuit 26 discriminates the valve of the output voltage V1' of the sensor 15 when the electrode 12 comes into contact with a fixed electrode 13 and, when the value is lower than a prescribed range, the circuit 26 output an 'H' output V7 and an OR element 27 outputs an abnormality signal H. When the value falls within the prescribed range, the circuit 26 outputs another 'H' output V6 and sends an operation start signal to a signal conversion processing circuit 17, resulting in a normal acceleration detecting state. In case the mobile electrode 12 does not come into contact with the fixed electrode 13 even when the value of the voltage V1' exceeds the prescribed range, a comparator 28 outputs an 'H' output and the OR element 27 outputs an abnormality signal. Thus the sensor 15 can be diagnosed by checking the continuity between both electrodes 12 and 13 while a certain voltage is applied across the electrodes 12 and 13. After the diagnosis is completed, a switch 24 is opened. As a result, the application of the voltage across the sensor 15 is canceled and both electrodes 12 and 13 are separated from each other.