A radiation plate structure includes a radiation plate, and a solder resist disposed on a main surface of the radiation plate and having at least one opening. The solder resist is made of any of polyimide (PI), polyamide (PA), polypropylene (PP), polyphenylene sulfide (PPS), a resin containing particulate ceramic (e.g., aluminum nitride (AlN), silicon nitride (Si3N4), or aluminum oxide (Al2O3)), and a high-melting-point insulator made of, for instance, glass.