An image forming apparatus includes an image forming unit, a controller including a master unit and a slave unit, and a serial communication line connecting the master unit and the slave unit and used in serial communication. The master unit includes an arithmetic module that generates first serial data to be transmitted to the slave unit to monitor serial communication, and monitors whether the first serial data has been successfully received in accordance with second serial data received from the slave unit. The slave unit includes a pulse signal output module that outputs pulse signals for a number of pulses which is set from the first serial data, an analog voltage output module that receives the pulse signals, changes an analog voltage in accordance with the number of pulses, and outputs the analog voltage, and an analog/digital conversion module that converts the analog voltage into the second serial data.