A condition monitoring system configured to monitor a condition of facilities by measuring vibration of the facilities, the condition monitoring system comprising: a vibration detector (170, 270, 270#) configured to output vibration detected by sensing of an acceleration generated in the facilities as voltage variation from a bias voltage; a voltage processing circuit configured to remove the bias voltage from an output voltage from the vibration detector (170, 270, 270#); a monitoring apparatus configured to monitor a condition of the facilities based on an output from the voltage processing circuit ; characterized by a connection circuit configured to connect an output of the vibration detector (170, 270, 270#) to the monitoring apparatus without the voltage processing circuit being interposed, the monitoring apparatus diagnosing the vibration detector (170, 270, 270#) as being abnormal when the output voltage received through the connection circuit is not included in a prescribed range including the bias voltage, wherein the connection circuit includes a relay connected in parallel to the voltage processing circuit , the monitoring apparatus is configured to control the relay to be conducting while abnormality of the vibration detector (170, 270, 270#) is being diagnosed, the vibration detector (170, 270, 270#) includes a piezoelectric element and a field effect transistor, FET , wherein the piezoelectric element is configured to output charges in accordance with a vibration acceleration, and the condition monitoring system further comprising a constant current source implemented by a DC power supply and a constant current element wherein a constant current is supplied from the constant current source implemented by the DC power supply and the constant current element through an output line to the FET , which is configured to regulate a voltage level on the output line to a prescribed bias voltage and wherein the FET has a gate connected to an output terminal of the piezoelectric element . a vibration detector (170, 270, 270#) configured to output vibration detected by sensing of an acceleration generated in the facilities as voltage variation from a bias voltage; a voltage processing circuit configured to remove the bias voltage from an output voltage from the vibration detector (170, 270, 270#); a monitoring apparatus configured to monitor a condition of the facilities based on an output from the voltage processing circuit ; characterized by a connection circuit configured to connect an output of the vibration detector (170, 270, 270#) to the monitoring apparatus without the voltage processing circuit being interposed, the monitoring apparatus diagnosing the vibration detector (170, 270, 270#) as being abnormal when the output voltage received through the connection circuit is not included in a prescribed range including the bias voltage, wherein the connection circuit includes a relay connected in parallel to the voltage processing circuit , the monitoring apparatus is configured to control the relay to be conducting while abnormality of the vibration detector (170, 270, 270#) is being diagnosed, the vibration detector (170, 270, 270#) includes a piezoelectric element and a field effect transistor, FET , wherein the piezoelectric element is configured to output charges in accordance with a vibration acceleration, and the condition monitoring system further comprising a constant current source implemented by a DC power supply and a constant current element wherein a constant current is supplied from the constant current source implemented by the DC power supply and the constant current element through an output line to the FET , which is configured to regulate a voltage level on the output line to a prescribed bias voltage and wherein the FET has a gate connected to an output terminal of the piezoelectric element . <EOS>