A power-saving type multi-band microwave detector is operable in a high sensitivity mode and a low sensitivity mode for achieving higher sensitivity for both non-modulated and modulated microwaves to make the detecting operation reliable without causing significant increase of power consumption. The power-saving type multi-band microwave detector includes a receiver circuit including a local oscillator for generating an oscillation output and means for sweeping the frequency of the oscillation output over a predetermined reception band, a power supply for periodically supplying a power for the receiver circuit to maintain the latter active while the power is supplied, and control circuit for controlling the receiver circuit and the power supply means for an intermittent microwave receiving operation. The control circuit operates the receiver circuit and the power supply in a first mode having a higher sensitivity and higher power consumption and a second mode having a lower sensitivity and lower power consumption, and the control means periodically performing the first mode at a lower frequency than the second mode.