PURPOSE: To improve a writing characteristic by forming a B region into a P type insular region surrounded by a buried layer and an insulating separation region through a thin oxide film and shaping a P type region deeper than the B region while using the oxide film narrowed as a mask.

CONSTITUTION: The N type buried layer 2 having high concentration is diffused to a P type substrate, an N type layer 3 is grown, and separated by P type layers 4, and an insular N type region 3' is formed. A thick oxide film 5 is shaped employing the Si nitride film 11 on the thin SiO2 layers 4 on the substrate as a mask, and the P type base region 6 is formed through the film 11. The P type region 13 deeper than the P type B region 6 is shaped through ion injection while using the SiO2 nitride film narrowed as the mask. A SiO2 film 14 shaped onto the region 13 is removed selectively while being thinned. Poly Si15 is attached to an E diffusion window section, impurities are diffused from the diffusion window section, an E region is formed, and a shallow junction is manufactured. Accordingly, the junction breakdown type PROM having an excellent writing characteristic can be produced.