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.