A diagnosis apparatus for a photo coupler having at least one photo coupler including a light emitting element the amount of light emitted from which changes in accordance with the value of a current flowing therethrough and a light receiving element which receives the light from the light emitting element to convert it into an electric signal corresponding to the received amount of light, a checking current setting circuit for setting a current flowing through the light emitting element to a current value for checking, a level detecting circuit for controlling a current flowing through the light emitting element on the basis of the current value set by the checking current setting circuit, a reference signal generator for generating a reference signal, a comparing and detecting circuit for comparing the electric signal from the light receiving element with the reference signal and detecting the deterioration of a function of the photo coupler on the basis of the result of comparison.