A sound source separation method using four-line directivity microphones mounted on a head of a robot is proposed and applied to speech recognition under existence of two disturbances of speech. Sound source separation methods using microphones mounted on robot heads generally used strict head-related transfer functions (HRTF). We propose a robust sound source separation that does not require an estimate of a strict HRTF. Our method takes advantage of a sound pressure difference with the robot head acting as a sound barrier. The enhancement of the difference in the target speech is performed by signal processing of three layers:two-line SAFIA, two-line Spectral Subtraction and their integration. The experimental results of three simultaneous utterance recognition with vocabulary of 20K show that the proposed method is effective in achieving 71% error reduction.