PURPOSE: To achieve a microwave circuit integration mixer having broad band, low noise and greater image suppression ratio, by providing a band-pass filter consisting of 1/4 wavelength end opened stub with a strip line transmitting a high frequency signal power to the mixer diode.

CONSTITUTION: A high frequency signal inputted from a high frequency signal input terminal 1 propagates on a main line 2 and is applied to a mixer diode 3. A local oscillation signal inputted from a local signal input terminal 4 is applied to the mixer diode 3 via a local signal band-pass filter 5 coupled with the main line 2 and an intermediate frequency signal is outputted through a low-pass filter 6. A band block filter 11 consisting of 3 stages of 1/4 wavelength tip opened stubs provided in 1/4 wavelength interval in common use of an image signal block filter and a local oscillation block filter, is provided at a location where the impedance viewed from the high frequency input terminal of the diode 3 at the main line 2 is short-circuited at the image frequency.