An ECU monitoring system includes: an ECU that includes an input interface unit, a computer unit, and an output interface unit; a storage unit that stores a normal input signal and a normal output signal; a comparing unit that calculates a compared difference by comparing the current input signal with a previous input signal stored in the storage unit, and outputs an alarming signal showing the current input signal is abnormal, when the compared difference is out of a predetermined range; and a circulator that receives the output signal, provides the output signal when the output signal is a normal output signal, and provides a normal output signal to the output interface unit, instead of the output signal outputted from the computer in response to the abnormal input signal, when receiving the alarming signal from the comparing unit.