PURPOSE: To set optional directivity for an antenna beam and to perform wide- area observation by providing a transmitter with a switching function and varying the amount of phase shifting of a variable phase shifter.

CONSTITUTION: A transmit signal is radiated from an antenna 7 through the variable phase shifter 6 whose phase-shifting amount is varied optionally. A receive signal reflected on the ground is inputted to a receiver 8. An SAR control part is equipped with a transmitter controller 9 which switches on and off state of (n) units of transmitters and sends out control signals and command signals to the transmitters, and a phase shifter controller 10 which calculates the amount of phase shifting of each variable phase shifter by using a computer 11 according to the command signal to obtain an optional antenna beam direction. Then, a receive output signal is recorded at a recording part 13 through a digital signal processing part 12 and the recording data is transmitted to an SAR picture processor 14 on the ground.