A keyboard circuit comprising: a plurality of switch groups (51-i, i=0∼3), each switch group (51-i) including three switches (SW0, SW1, SW2), each switch group (51-i) being provided so as to correspond to one of a plurality of keys, the switches having ON/OFF states that change in response to a key-pressing/key-releasing operation of the corresponding key; characterized in that each switch group (51-i) further includes three transistors (TRk, k=0∼2), each transistor having at least three input/output terminals, wherein a signal input to a first terminal can control the ON/OFF state of a current between a second terminal and a third terminal, and each of the switches (SW0, SW1, SW2) is connected to a respective one of the third terminals; and by further comprising: a first wire group including three wires, each of the three wires in the first wire group is configured to supply a control signal (KBk, k=0∼2) to a respective one of the first terminals of each of the plurality of switch groups to select a switch among the three switches in the switch group, a second wire group that supplies scan output signals (KEi, i=0∼3) to the second terminals of each of the plurality of switch groups, and a third wire group that provides input signals (KIj, j=0∼21) indicating the ON/OFF state of the switches; wherein the keyboard circuit is adapted to control the control signals (KBk, k=0∼2) and the scan output signals (KEi, i=0∼3) so that the control signals turn ON the transistors (TRk) in order and to detect the input signals (KIj) indicating the ON/OFF state of the respective switches on the third wire group in the order by supplying the scan output signals to the switches in the order, thereby detecting the state of the switches based on a combination of states of the control signals (KBk, k=0∼2), the scan output signals (KEi, i=0∼3), and the input signals (KIj, j=0∼21). a plurality of switch groups (51-i, i=0∼3), each switch group (51-i) including three switches (SW0, SW1, SW2), each switch group (51-i) being provided so as to correspond to one of a plurality of keys, the switches having ON/OFF states that change in response to a key-pressing/key-releasing operation of the corresponding key; characterized in that each switch group (51-i) further includes three transistors (TRk, k=0∼2), each transistor having at least three input/output terminals, wherein a signal input to a first terminal can control the ON/OFF state of a current between a second terminal and a third terminal, and each of the switches (SW0, SW1, SW2) is connected to a respective one of the third terminals; and by further comprising: a first wire group including three wires, each of the three wires in the first wire group is configured to supply a control signal (KBk, k=0∼2) to a respective one of the first terminals of each of the plurality of switch groups to select a switch among the three switches in the switch group, a second wire group that supplies scan output signals (KEi, i=0∼3) to the second terminals of each of the plurality of switch groups, and a third wire group that provides input signals (KIj, j=0∼21) indicating the ON/OFF state of the switches; a first wire group including three wires, each of the three wires in the first wire group is configured to supply a control signal (KBk, k=0∼2) to a respective one of the first terminals of each of the plurality of switch groups to select a switch among the three switches in the switch group, a second wire group that supplies scan output signals (KEi, i=0∼3) to the second terminals of each of the plurality of switch groups, and a third wire group that provides input signals (KIj, j=0∼21) indicating the ON/OFF state of the switches; wherein the keyboard circuit is adapted to control the control signals (KBk, k=0∼2) and the scan output signals (KEi, i=0∼3) so that the control signals turn ON the transistors (TRk) in order and to detect the input signals (KIj) indicating the ON/OFF state of the respective switches on the third wire group in the order by supplying the scan output signals to the switches in the order, thereby detecting the state of the switches based on a combination of states of the control signals (KBk, k=0∼2), the scan output signals (KEi, i=0∼3), and the input signals (KIj, j=0∼21).