Key switches SW1-SW6 short-circuit one ends of resistors R corresponding thereto and a termination of a resistor string unit STR1, in accordance with inputs made by keys KEY1-KEY6 assigned thereto, respectively. The keys KEY1-KEY6 are made active in a period when the selection command SEL is at “P1”, so that control unit 2 can identify which key is used to make an input, out of the keys KEY1-KEY6. The keys KEY7-KEY12 are made active in a period when the selection command SEL is at “P2”, so that control unit 2 can identify which key is used to make an input, out of the keys KEY7-KEY12.