An onboard electronic device includes: an element that generates heat; a member that is provided between the element and a coolant cooling the element, and differs in thermal expansion coefficient from the element; an element temperature sensor that detects the temperature of the element; a coolant temperature sensor that detects the temperature of the coolant; and a controller that controls operation of the element such that the temperature of the element allowed when the temperature of the coolant is a first temperature is lower than the temperature of the element allowed when the temperature of the coolant is a second temperature that is higher than the first temperature.