A semiconductor integrated circuit device has a p-type substrate to which a ground voltage is applied and a floating-type NMOSFET which is integrated on the p-type substrate and to which a negative voltage lower than the ground voltage is applied. The floating-type NMOSFET includes an n-type buried layer buried in the p-type substrate, a high voltage n-type well formed on the n-type buried layer and floats electrically, a p-type drift region formed in the n-type well, an n-type drain region and an-type source region formed in the p-type drift region, and a gate electrode formed on a channel region interposed between the n-type drain region and the n-type source region. The high voltage n-type well includes an n-type tunnel region, with a higher impurity concentration than that of the high voltage n-type well, inside a peripheral region formed so as to surround the p-type drift region.