A moving body detector, comprising: an oscillator configured to output an oscillation signal having a prescribed frequency; a transmitter configured to transmit, to a space to be monitored, continuous energy waves of which amplitude is periodically changed according to the oscillation signal outputted from the oscillator; a receiver configured to output a receiving signal, when receiving a reflected wave generated due to reflection of the continuous energy waves by an object that exists within the space to be monitored and/or when receiving directly a part of the waves output from the transmitter; a phase-detector configured to mix the oscillation signal and the receiving signal to obtain a mixed signal, the mixed signal representing a frequency difference between the oscillation signal and the receiving signal; a detector configured to detect a moving body within the space to be monitored, by processing the mixed signal, to output a detection signal; and a controller configured to control the oscillator, transmitter, receiver, phase-detector and detector, and alternatively switch a moving body detection mode of detecting the moving body and a malfunction detection mode of detecting presence or absence of malfunction occurrence, wherein the controller is configured to control, in the moving body detection mode, to input the oscillation signal to the transmitter and the phase-detector, and make the phase-detector output the mixed signal according to the frequency difference between the oscillation signal and the receiving signal, characterized in that the controller is configured to compare, in the malfunction detection mode, the output of the phase-detector obtained when controlling to input the oscillation signal to only the phase-detector, with an output of the phase-detector obtained after shifting from the state of inputting the oscillation signal to only the phase-detector to a state of inputting the oscillation signal also to the transmitter in addition to the phase-detector, to detect the presence or absence of the malfunction occurrence; wherein the controller is configured to determine that there is no malfunction in the transmitter and receiver if a change of the signal level is equal to or more than a prescribed threshold, and to determine that that there is a malfunction in the transmitter or the receiver, if the change of the signal level is less than the prescribed threshold. an oscillator configured to output an oscillation signal having a prescribed frequency; a transmitter configured to transmit, to a space to be monitored, continuous energy waves of which amplitude is periodically changed according to the oscillation signal outputted from the oscillator; a receiver configured to output a receiving signal, when receiving a reflected wave generated due to reflection of the continuous energy waves by an object that exists within the space to be monitored and/or when receiving directly a part of the waves output from the transmitter; a phase-detector configured to mix the oscillation signal and the receiving signal to obtain a mixed signal, the mixed signal representing a frequency difference between the oscillation signal and the receiving signal; a detector configured to detect a moving body within the space to be monitored, by processing the mixed signal, to output a detection signal; and a controller configured to control the oscillator, transmitter, receiver, phase-detector and detector, and alternatively switch a moving body detection mode of detecting the moving body and a malfunction detection mode of detecting presence or absence of malfunction occurrence, wherein the controller is configured to control, in the moving body detection mode, to input the oscillation signal to the transmitter and the phase-detector, and make the phase-detector output the mixed signal according to the frequency difference between the oscillation signal and the receiving signal, characterized in that the controller is configured to compare, in the malfunction detection mode, the output of the phase-detector obtained when controlling to input the oscillation signal to only the phase-detector, with an output of the phase-detector obtained after shifting from the state of inputting the oscillation signal to only the phase-detector to a state of inputting the oscillation signal also to the transmitter in addition to the phase-detector, to detect the presence or absence of the malfunction occurrence; wherein the controller is configured to determine that there is no malfunction in the transmitter and receiver if a change of the signal level is equal to or more than a prescribed threshold, and to determine that that there is a malfunction in the transmitter or the receiver, if the change of the signal level is less than the prescribed threshold.