A vehicular electronic control device activated according to an on-state signal from a main switch and a wake-up signal from an external device includes: a microcomputer; a voltage generating circuit for the microcomputer; a start circuit that generates the operation voltage with the voltage generating circuit when the on-state signal or the wake-up signal input; a printed wiring board; and a transmission wiring pattern that transmits the on-state signal. A first region of the board mounts the microcomputer and the voltage generating circuit, which activate the vehicular electronic control device when the main switch turns on. A second region of the board mounts the start circuit, which activates the vehicular electronic control device according to the wake-up signal. The transmission wiring pattern extends from the first region to reach the second region.