A diagnostic apparatus includes a control device and an extension device connectable to the control device. The control device includes one or more cutoff blocks that switch between allowing and cutting off power supply to a load, a first diagnostic pulse generator that generates a diagnostic signal indicating that the power to the load is to be cut off, a first cutoff diagnosing device that detects a supply voltage to the load, and an extension device detector that determines whether the extension device is connected to the control device. The extension device includes a second diagnostic pulse generator that generates a diagnostic signal, and a second cutoff diagnosing device that detects a supply voltage to the load.