Microbubbles insonified by ultrasound show nonlinear oscillation and reflect waves including harmonic and subharmonic signals. Subharmonic imaging can be more useful, though occurrence conditions of subharmonics are not clarified enough. In this study, we investigated effects of microbubble interaction on occurrence of subharmonics. The interaction depends on the distance between two microbubbles and their radius ratios. For different values of these parameters, we numerically calculated the Rayleigh-Plesset equations with these interaction terms added and investigated occurrence conditions of subharmonics. Obtained results show that, by decreasing the distance between the bubbles and increasing the radius of one of the bubbles, the threshold of the pressure amplitude and the transmit frequency for generating subharmonics become lower.