A machine state monitoring device is a machine state monitoring device that is intended for a motor drive device of a machine tool. The motor drive device includes: a motor; a converter and an inverter that adjust a power supply voltage; a power failure detection unit that acquires an input voltage of the motor drive device so as to detect a power failure in the motor drive device; and a communication unit that transmits, based on the result of the detection of the detection unit, a time when the power failure is started, and a time when the power failure is completed. The machine state monitoring device includes: a communication unit that receives information of the time when the power failure is started and the time when the power failure is completed from the communication unit of the motor drive device; and a storage unit that stores the information.