A diagnosis apparatus includes: a sensor that detects diagnosis target information generated by a diagnosis target device; a threshold setting unit that sets a threshold for the diagnosis target information; and a diagnosis unit that diagnoses the diagnosis target device based on the diagnosis target information detected by the sensor and the threshold, in which the threshold setting unit sets the threshold based on the diagnosis target information in a predetermined period before a diagnosis time point, and the diagnosis unit performs diagnosis based on a current time point threshold set at the diagnosis time point and at least one past threshold set in the past.