connection determination section 5151 the switch control section 5152 switches on the power source changing over switch 512 here the power source control section 5153 controls the duty ratio of the pwm signal applied to the switching element 5111 so that the power supply voltage rises gradually thereby gently raising the power supply voltage further the voltage value comparison section 5154 makes the adc 514 measure the voltage value v fig4 a is a graph of the power supply voltage in the related art and fig4 b is a graph of the power supply voltage controlled by the power source control section 5153 so that the voltage rises gradually when the power source changing over switch 512 is switched on the power source control section 5153 gradually raises the power supply voltage supplied to the audio signal line 56 and fixes it to be a predetermined voltage after time t has elapsed and therefore the rapid rise of the power supply voltage shown in fig4 a does not occur when the power source changing over switch 512 is switched on the voltage value comparison section 5154 compares the voltage value v obtained from the adc 514 with the reference voltage value v 0 stored in the memory 5155 since the impedance value of the dynamic microphone which is several hundreds u03c9 for example and the impedance value of the condenser microphone which is several tens k u03c9 for example are significantly different from each other it.