An electronic substrate has a tabular base material which can install a heater element and a cooling structure that cools the heater element. The electronic substrate can be plugged in/out in a case in the direction which is almost parallel to the face of the base material. The cooling structure has a first heat radiation part with a hollow shape and a heat transfer part. The first heat radiation part radiates the generated heat of a heater element installed in the base material. A heat transfer part transfers the generated heat to the first heat radiation part. The first heat radiation part has a first joint surface formed along a face which is almost vertical to the insert and removal direction of the base material. The first heat radiation part is connected to a second radiation part set up in the case thermally through the first joint surface.