Microwave coagulation therapy (MCT) has been used mainly for the treatment of small-size tumors. The operating frequency is 2450 MHz for the present MCT. In conventional MCT antennas, there exists a problem that the size of the coagulated region is insufficient. In this paper, the authors analyzed the heating characteristics of an array applicator composed of two coaxial-slot antennas by using computer simulation. The validity of the analysis was confirmed by the experiment using the liver of a pig. Moreover, the authors investigated the relation between the array spacing and heating volume produced by the two-antenna array applicator. As a result, the coagulated region required in the MCT could almost be achieved under the conditions that the net input power of each antenna was 50 W and the array spacing was 10 mm.