PURPOSE: To increase the SN ratio, i.e., the ratio of the level of a voice signal to the level of peripheral noises, by disposing two microphones having the same characteristics with a proper spacing from each other.

CONSTITUTION: A pre-processing circuit for a voice recognizing section is constructed such that the output signals f1, f2 of two microphones 2c, 2d are applied as input signals to a differential amplifier 4 and the output signal f0 of the differential amplifier 4 is applied as an input signal to the voice recognizing section. Here, it is provided that the distance between a peripheral noise source GN and the microphones is dN and the levels of peripheral noises received by the microphones 2c, 2d are respectively N1, N2. Further, it is provided that the distance between a voice source Gs and the microphones is dS and the levels of the voice signal received by the microphones 2c, 2d are respectively S1, S2. Providing that the following relationships, N0=N2-N1, S0=S2-S1 and dM dN, are satisfied, it is assumed that N1 is almost equal to N2. Further, by using microphones having a high directivity and providing that dM is far greater than dS, S0 is substantially equal to S2. Under such conditions, the SN ratio is obtained from the following equation, S0/N0=(S2-S1)/(N2-N1), thereby the SN ratio is increased.