A machine tool controller capable of identifying the factors of a warning easily is provided. A controller of a machine tool includes a power supply monitoring unit, a power supply abnormality detecting unit, and an abnormality diagnosis unit. The power supply monitoring unit acquires measurements indicating an operating state of a machine tool and/or a peripheral device (for example, an AC power supply or a motor) of the machine tool. The power supply abnormality detecting unit detects an abnormality in the operating state of the machine tool and/or the peripheral device and outputs a signal indicating the abnormality and the measurements obtained when the abnormality was detected. The abnormality diagnosis unit automatically selects a measurement related to the abnormality among the measurements and automatically diagnoses factors of the abnormality.