A motor driving device that accurately achieves power failure detection according to a power failure tolerance with a relatively simple configuration. A counter input computing unit determines, as a counter input value, a value that is inversely proportional to the power failure tolerance determined from a voltage amplitude value and supplies the counter input value to a counter. The counter accumulates the input value at predetermined intervals and outputs an output value. A comparator determines that power failure occurs if the output of the counter 42 exceeds a threshold value.