PURPOSE: To obtain an excellent image without degrading receiving sensitivity, in a microwave radiometer mounted on an artifical satellite and the like, by making the width of the beam of an antenna in the advancing direction larger than the width of the beam in the antenna sweeping direction.

CONSTITUTION: In a microwave radiometer, which is mounted on an artificial satellite and the like and performs remote sensing, the electromagnetic wave in a microwave band, which is transmitted from the ground surface, is received by an antenna 1. The antenna 1 is swept by an antenna driving part 2 and observes the broad ground surface. The output voltage, which is amplified and detected by a receiving part 3 and corresponds to the temperature of the antenna, is smoothed by a charge and discharge integrator 4 and transmitted to the ground by a transmitting part 6 through an A/D converter 5. The beam of the antenna 1 is formed by the following way in consideration of the ground surface distance which is swept by the antenna for the integrating time: the width of the beam in the advancing direction of the satellite is larger than the width of the beam in the direction of the antenna sweep. Thus the excellent image can be obtained without degarding the receiving sensitivity and without the correction by software on the ground.