A diagnostic apparatus comprising: a memory coupled to a processor, the memory storing instructions that when executed by the processor configure the processor to: acquire sensor values from sensors on a device (2, 3) and acquire the associated operational states of the device (2, 3) from other sensors at a plurality of times of operation states at which the device (2, 3) is known to be normal or abnormal; a reference data creation unit configured to start an operation of the device (2, 3) and to repeat a process of storing, in a new record in a storage unit, the sensor values acquired from the device (2, 3) and the respective operation states while changing variously the operation states of the device (2, 3) at the different reference times in association with each of the determined operation states until there is no non-corresponding operation state with which the acquired sensor value is not yet associated; a diagnostic data creation unit configured to acquire, at a diagnosis time at which it is not known whether the device (2, 3) is normal or abnormal, an operation state and a sensor value of the device (2, 3) at the diagnosis time; and a diagnosis unit configured to read the sensor value associated with the acquired operation state from the storage unit and to compare the sensor value acquired at the diagnosis time with the read sensor value to display a result of determination on whether the device (2, 3) at the diagnosis time is normal or abnormal, wherein the sensor value associated with the acquired operation state is a value obtained by representing a feature amount of a measurement value measured by a sensor in a vector form, and the diagnosis unit is configured to compare a diagnosis target feature amount vector representing the sensor value acquired at the diagnosis time with a reference feature amount vector representing the read sensor value. acquire sensor values from sensors on a device (2, 3) and acquire the associated operational states of the device (2, 3) from other sensors at a plurality of times of operation states at which the device (2, 3) is known to be normal or abnormal; a reference data creation unit configured to start an operation of the device (2, 3) and to repeat a process of storing, in a new record in a storage unit, the sensor values acquired from the device (2, 3) and the respective operation states while changing variously the operation states of the device (2, 3) at the different reference times in association with each of the determined operation states until there is no non-corresponding operation state with which the acquired sensor value is not yet associated; a diagnostic data creation unit configured to acquire, at a diagnosis time at which it is not known whether the device (2, 3) is normal or abnormal, an operation state and a sensor value of the device (2, 3) at the diagnosis time; and a diagnosis unit configured to read the sensor value associated with the acquired operation state from the storage unit and to compare the sensor value acquired at the diagnosis time with the read sensor value to display a result of determination on whether the device (2, 3) at the diagnosis time is normal or abnormal, wherein the sensor value associated with the acquired operation state is a value obtained by representing a feature amount of a measurement value measured by a sensor in a vector form, and the diagnosis unit is configured to compare a diagnosis target feature amount vector representing the sensor value acquired at the diagnosis time with a reference feature amount vector representing the read sensor value.