A diagnostic apparatus, comprising: a control device; and an extension device connectable to the control device, wherein the control device includes: a cutoff block configured to switch between allowing and cutting off power supply to a load; a first diagnostic pulse generator configured to generate a first diagnostic signal indicating that the power supply to the load is to be cut off; a first cutoff diagnosing device configured to detect a supply voltage to the load; and an extension device detector configured to determine whether the extension device is connected to the control device, the extension device includes: a second diagnostic pulse generator configured to generate a second diagnostic signal indicating that the power supply to the load is to be cut off; and a second cutoff diagnosing device configured to detect a supply voltage to the load, when the extension device detector has determined that the extension device is not connected to the control device, the first diagnostic pulse generator is configured to output the first diagnostic signal to the cutoff block, the cutoff block is configured to cut off the power supply to the load in response to the first diagnostic signal output from the first diagnostic pulse generator, and the first cutoff diagnosing device is configured to diagnose whether there is a failure in the cutoff block based on the supply voltage to the load that is detected, and when the extension device detector has determined that the extension device is connected to the control device, the second diagnostic pulse generator is configured to output the second diagnostic signal to the cutoff block, the cutoff block is configured to cut off the power supply to the load in response to the second diagnostic signal output from the second diagnostic pulse generator, and the second cutoff diagnosing device is configured to diagnose whether there is a failure in the cutoff block based on the supply voltage to the load that is detected. a control device; and an extension device connectable to the control device, wherein the control device includes: a cutoff block configured to switch between allowing and cutting off power supply to a load; a first diagnostic pulse generator configured to generate a first diagnostic signal indicating that the power supply to the load is to be cut off; a first cutoff diagnosing device configured to detect a supply voltage to the load; and an extension device detector configured to determine whether the extension device is connected to the control device, a cutoff block configured to switch between allowing and cutting off power supply to a load; a first diagnostic pulse generator configured to generate a first diagnostic signal indicating that the power supply to the load is to be cut off; a first cutoff diagnosing device configured to detect a supply voltage to the load; and an extension device detector configured to determine whether the extension device is connected to the control device, the extension device includes: a second diagnostic pulse generator configured to generate a second diagnostic signal indicating that the power supply to the load is to be cut off; and a second cutoff diagnosing device configured to detect a supply voltage to the load, a second diagnostic pulse generator configured to generate a second diagnostic signal indicating that the power supply to the load is to be cut off; and a second cutoff diagnosing device configured to detect a supply voltage to the load, when the extension device detector has determined that the extension device is not connected to the control device, the first diagnostic pulse generator is configured to output the first diagnostic signal to the cutoff block, the cutoff block is configured to cut off the power supply to the load in response to the first diagnostic signal output from the first diagnostic pulse generator, and the first cutoff diagnosing device is configured to diagnose whether there is a failure in the cutoff block based on the supply voltage to the load that is detected, and the first diagnostic pulse generator is configured to output the first diagnostic signal to the cutoff block, the cutoff block is configured to cut off the power supply to the load in response to the first diagnostic signal output from the first diagnostic pulse generator, and the first cutoff diagnosing device is configured to diagnose whether there is a failure in the cutoff block based on the supply voltage to the load that is detected, and when the extension device detector has determined that the extension device is connected to the control device, the second diagnostic pulse generator is configured to output the second diagnostic signal to the cutoff block, the cutoff block is configured to cut off the power supply to the load in response to the second diagnostic signal output from the second diagnostic pulse generator, and the second cutoff diagnosing device is configured to diagnose whether there is a failure in the cutoff block based on the supply voltage to the load that is detected. the second diagnostic pulse generator is configured to output the second diagnostic signal to the cutoff block, the cutoff block is configured to cut off the power supply to the load in response to the second diagnostic signal output from the second diagnostic pulse generator, and the second cutoff diagnosing device is configured to diagnose whether there is a failure in the cutoff block based on the supply voltage to the load that is detected.