On a semiconductor substrate of P-type silicon, an active area including a channel forming region with a smaller dimension along the gate width and a source region and a drain region extending along the gate length is formed so as to be surrounded with an isolation area of an insulating oxide film. On the isolation area on the semiconductor substrate and the channel forming region of the active area, a gate electrode is formed with a gate insulating oxide film sandwiched therebetween. A channel lower insulating layer is formed, out of the same insulating oxide film for the isolation area, merely in an area below the channel forming region below the gate electrode in the active area of the semiconductor substrate.