PURPOSE: To subtract noise generated within a receiver and to improve resolution of an A/D converter by providing a differential amplifier and a sample holding circuit subsequent to an integration circuit in the total power type receiver for a microwave radiometer.

CONSTITUTION: An electromagnetic wave received by a receiving antenna 1 is sent through a measurement/calibration changing over switch 2 to an RF/IF circuit 3 for frequency conversion. Square-law detected 4 output is integrated 6 and then applied to a differential amplifier 11. First, the switch 2 turned to the noise source side for calibration, and a noise source switch 10 for calibration is turned to the side of a low temperature noise source 8. At this time, a switch 13 is closed and a switch 14 is opened, so that a signal is held in a sample holding circuit 12. Then, the switch 10 is changed over to the side of a high temperature noise source 9. And the switch 13 is opened and the switch 14 is closed, so that an attenuator 5 is adjusted by an AGC circuit composed of the stepwise attenuator 5 and an A/D converter 7. After such calibration, the switch is changed over to the side of the antenna 1. In this state, the objective can be observed and internal noise can be subtracted.