An open phase and interruption detection device for a three-phase AC power supply, the device comprising: three photocouplers (14, 24, 34) that are configured to be connected between respective lines of an L1 phase and an L2 phase, the L2 phase and an L3 phase, and the L3 phase and the L1 phase, respectively, of the three-phase AC power supply, each includes a light receiving element (14b, 24b, 34b) that is configured to turn on at a predetermined threshold voltage or higher, and are configured to convert line voltages of the L1 phase and the L2 phase, the L2 phase and the L3 phase, and the L3 phase and the L1 phase of the three-phase AC power supply into three detection signals; and a controller that is configured to acquire three pulse detection signals from the three detection signals output from the three photocouplers (14, 24, 34) or an inverted pulse detection signal obtained by inverting the three pulse detection signals, wherein the controller is configured to perform an AND operation of the three pulse detection signals or is configured to invert three inverted pulse detection signals and to perform the AND operation, to generate an AND operation pulse, and is configured to determine a normal state and presence or absence of an open phase or interruption by the AND operation pulse; characterized in that: the controller includes first and second counters, and the controller a) is configured to change the first counter sequentially in a constant direction during a time in which a value of the AND operation pulse continues to be 0, and is configured to determine that it is a normal state when the first counter exceeds a first value that is set in advance, and b) is configured to return the first counter to 0 when the value of the AND operation pulse is switched from 0 to 1 and is configured to sequentially change the second counter in a constant direction during a time in which the AND operation pulse continues to be 1, and is configured to determine the presence of the interruption when it is determined that the second counter is equal to or exceeds a second value that is set in advance, or is configured to determine the presence of the open phase when it is determined that the second counter is equal to or exceeds a third value smaller than the first and second values. three photocouplers (14, 24, 34) that are configured to be connected between respective lines of an L1 phase and an L2 phase, the L2 phase and an L3 phase, and the L3 phase and the L1 phase, respectively, of the three-phase AC power supply, each includes a light receiving element (14b, 24b, 34b) that is configured to turn on at a predetermined threshold voltage or higher, and are configured to convert line voltages of the L1 phase and the L2 phase, the L2 phase and the L3 phase, and the L3 phase and the L1 phase of the three-phase AC power supply into three detection signals; and a controller that is configured to acquire three pulse detection signals from the three detection signals output from the three photocouplers (14, 24, 34) or an inverted pulse detection signal obtained by inverting the three pulse detection signals, wherein the controller is configured to perform an AND operation of the three pulse detection signals or is configured to invert three inverted pulse detection signals and to perform the AND operation, to generate an AND operation pulse, and is configured to determine a normal state and presence or absence of an open phase or interruption by the AND operation pulse; the controller includes first and second counters, and the controller a) is configured to change the first counter sequentially in a constant direction during a time in which a value of the AND operation pulse continues to be 0, and is configured to determine that it is a normal state when the first counter exceeds a first value that is set in advance, and b) is configured to return the first counter to 0 when the value of the AND operation pulse is switched from 0 to 1 and is configured to sequentially change the second counter in a constant direction during a time in which the AND operation pulse continues to be 1, and is configured to determine the presence of the interruption when it is determined that the second counter is equal to or exceeds a second value that is set in advance, or is configured to determine the presence of the open phase when it is determined that the second counter is equal to or exceeds a third value smaller than the first and second values. a) is configured to change the first counter sequentially in a constant direction during a time in which a value of the AND operation pulse continues to be 0, and is configured to determine that it is a normal state when the first counter exceeds a first value that is set in advance, and b) is configured to return the first counter to 0 when the value of the AND operation pulse is switched from 0 to 1 and is configured to sequentially change the second counter in a constant direction during a time in which the AND operation pulse continues to be 1, and is configured to determine the presence of the interruption when it is determined that the second counter is equal to or exceeds a second value that is set in advance, or is configured to determine the presence of the open phase when it is determined that the second counter is equal to or exceeds a third value smaller than the first and second values.