An electronic control unit mounted in an object includes: a rewritable memory of control and reprogramming software; a not-always-on power supply system micro-computer operating in control and reprogramming modes and connected/disconnected with a power source; and an always-on power supply system micro-computer operating in the control and reprogramming modes and constantly connected with the power source. Each micro-computer includes: a first mode determination device determining whether to operate in an on-board mode when the power source starts to supply the electric power; and a second mode determination device determining whether to operate in the reprogramming mode. The always-on power supply system micro-computer includes a first transition device that detects a change in a power supply state of the not-always-on power supply system micro-computer. The first transition device switches a processing to a determination executed by the first mode determination device when the power supply state has changed.