In a method for producing an electronic part mounting substrate wherein a heat sinking metal base plate 12 is bonded to one side of a ceramic substrate 10, and one side of a circuit forming metal plate 14 is bonded to the other side thereof, an electronic part 16 being mounted on the other side of the circuit forming metal plate 14, the ceramic substrate 10 and the electronic part 16 are arranged in a mold so that the ceramic substrate 10 is spaced from the electronic part 16, and then, a molten metal is injected into the mold so that the molten metal contacts both sides of the ceramic substrate 10 and the electronic part 16. Then, the mold is cooled to solidify the molten metal, to form a heat sinking metal base plate on one side of the ceramic substrate 10 so that the heat sinking metal base plate is bonded directly to the one side of the ceramic substrate 10, and to form a circuit forming metal plate on the other side of the ceramic substrate 10 so that one side of the circuit forming metal plate is bonded directly to the other side of the ceramic substrate 10. When the circuit forming metal plate is formed, the electronic part is bonded directly to the other side of the circuit forming metal plate.