A diagnostic apparatus (10) in which a drive circuit (2) is configured of switching means (3) for supplying an electric current to a linear solenoid valve and variably regulating a valve opening degree on the basis of inputted ON-OFF signals, current detection means (4) for detecting an electric current flowing from the switching means (3) to the linear solenoid valve (1), and control means (5) for outputting ON-OFF signals to the switching means (3) on the basis of a detected current value from the current detection means (4) so that the linear solenoid valve has a target opening degree, and which diagnoses a ground short between the drive circuit (2) and the linear solenoid valve (1), the diagnostic apparatus including ground short detection means (5) for detecting a switching of ON-OFF signals inputted in the switching means (3) and determining a ground short fault when the number of ON-OFF switching cycles per a predetermined interval is equal to or less than a predetermined number of cycles.