To detect an abnormal state positively and to improve reliability by using an output signal being outputted from an oscillation circuit part, performing a predetermined signal processing to an electrical signal being converted by a sensor part, monitoring a signal outputted from the oscillation circuit part, and outputting a failure detection signal when a signal failure is detected.

A physical quantity sensor 1 comprises, for example, a sensor part 2, a signal-processing circuit part 4, an oscillation circuit part 5, and a self-diagnosis circuit part 6. The oscillation circuit part 5 outputs a sinusoidal wave signal with a reference frequency (a reference frequency signal) to the sensor part 2, a signal detection circuit part 3, and a self-diagnosis circuit part 6. The sensor part 2 converts a detected physical quantity to an electrical signal, based on the frequency of a reference frequency signal inputted from the oscillation circuit part 5, and outputs it to the signal detection circuit part 3. The signal-processing circuit part 4 amplifies a signal inputted from the signal detection circuit part 3 and outputs it to a control circuit 8. A reference frequency signal outputted from the oscillation circuit part 5 is monitored by the self-diagnosis circuit part 6, thus detecting whether the frequency of the frequency signal is within a predetermined range or not.