An electronic component mounting substrate includes an insulating base having a rectangular shape in plan view and including a first main surface, a second main surface facing the first main surface, and a recess open on the first main surface, a band-shaped metal layer on a sidewall of the recess, and an electrode extending from a bottom surface of the recess into the insulating base. The electrode has an end disposed in the insulating base, and the end includes an inclined portion inclined toward the second main surface.