A moving object detection system, comprising: an oscillation means (11 and 12) configured to generate a first oscillation signal (S 1) having a first frequency (f 1); a transmitter configured to transmit energy waves to a monitoring space, said energy waves having the same frequency as that of the first oscillation signal (S 1); a receiver configured to receive incoming energy waves from the monitoring space to generate an input signal (S in); a detector configured to obtain a detection signal from the first oscillation signal (S 1) and the input signal (S in), the detection signal being a Doppler shift signal when the input signal (S in) includes a second frequency (S 2) that is different from the first frequency (S 1); and a judgment device configured: to judge whether or not a moving object approaching or leaving the receiver exists in the monitoring space based on the detection signal; and to output a moving object detection signal if the moving object exists in the monitoring space; characterized in that the oscillation means (11 and 12) comprises: a first oscillator configured to generate the first oscillation signal (S 1) to supply the first oscillation signal (S 1) to the transmitter and the switch; and a second oscillator configured to generate a check signal (S c) to supply the check signal (Sc) to the switch, said check signal (S c) having a check frequency (f c) that is different from the first frequency (f 1), wherein: the moving object detection system is installed so as to monitor a monitoring space comprising an object which remains stationary with respect to the transmitter and the receiver; or the transmitter has the directivity by which the receiver can directly receive one of the ultrasonic waves from the transmitter;or the moving object detection system further includes a movable mechanism for changing the orientation of the transmitter so that the receiver can receive the ultrasonic waves from the transmitter, wherein the moving object detection system further comprises: a switch configured to supply the first oscillation signal (S 1) or the check signal (S c) to the detector; and a controller that has a monitoring phase and a check phase and is configured: to supply the first oscillation signal (f 1) to the detector through the switch at the monitoring phase; and also to supply the check signal (S c) to the detector through the switch at the check phase. an oscillation means (11 and 12) configured to generate a first oscillation signal (S 1) having a first frequency (f 1); a transmitter configured to transmit energy waves to a monitoring space, said energy waves having the same frequency as that of the first oscillation signal (S 1); a receiver configured to receive incoming energy waves from the monitoring space to generate an input signal (S in); a detector configured to obtain a detection signal from the first oscillation signal (S 1) and the input signal (S in), the detection signal being a Doppler shift signal when the input signal (S in) includes a second frequency (S 2) that is different from the first frequency (S 1); and a judgment device configured: to judge whether or not a moving object approaching or leaving the receiver exists in the monitoring space based on the detection signal; and to output a moving object detection signal if the moving object exists in the monitoring space; characterized in that the oscillation means (11 and 12) comprises: a first oscillator configured to generate the first oscillation signal (S 1) to supply the first oscillation signal (S 1) to the transmitter and the switch; and a second oscillator configured to generate a check signal (S c) to supply the check signal (Sc) to the switch, said check signal (S c) having a check frequency (f c) that is different from the first frequency (f 1), wherein: the moving object detection system is installed so as to monitor a monitoring space comprising an object which remains stationary with respect to the transmitter and the receiver; or the transmitter has the directivity by which the receiver can directly receive one of the ultrasonic waves from the transmitter;or the moving object detection system further includes a movable mechanism for changing the orientation of the transmitter so that the receiver can receive the ultrasonic waves from the transmitter, the moving object detection system is installed so as to monitor a monitoring space comprising an object which remains stationary with respect to the transmitter and the receiver; or the transmitter has the directivity by which the receiver can directly receive one of the ultrasonic waves from the transmitter;or the moving object detection system further includes a movable mechanism for changing the orientation of the transmitter so that the receiver can receive the ultrasonic waves from the transmitter, wherein the moving object detection system further comprises: a switch configured to supply the first oscillation signal (S 1) or the check signal (S c) to the detector; and a controller that has a monitoring phase and a check phase and is configured: to supply the first oscillation signal (f 1) to the detector through the switch at the monitoring phase; and also to supply the check signal (S c) to the detector through the switch at the check phase. a switch configured to supply the first oscillation signal (S 1) or the check signal (S c) to the detector; and a controller that has a monitoring phase and a check phase and is configured: to supply the first oscillation signal (f 1) to the detector through the switch at the monitoring phase; and also to supply the check signal (S c) to the detector through the switch at the check phase.