A MOS type semiconductor device is provided which includes a series Zener diode array for overvoltage protection, which is provided between source regions and an electrode having substantially the same potential as a drain electrode, and a field insulating film on which the series Zener diode array is provided. The thickness T (.mu.m) of the field insulating film is determined as a function of the clamp voltage V.sub.CE (V) of the series Zener diode array, such that the thickness T is held in the range as represented by: T.gtoreq.2.0.times.10.sup.-3 .times.V.sub.CE. The width W.sub.1 (.mu.m) of a portion of a second-conductivity-type isolation well that is close to the field insulating film on which the series Zener diode array is provided, and the width W.sub.2 (.mu.m) of a portion of the second-conductivity-type isolation well that is close to the field insulating film on which the series Zener diode array is not provided, are determined as a function of the clamp voltage V.sub.CE of the series Zener diode array, such that the widths W.sub.1, W.sub.2 are held in respective ranges as represented by: W.sub.1 .gtoreq.0.15 V.sub.CE, and W.sub.2 .gtoreq.0.05 V.sub.CE. By controlling the widths W.sub.1, W.sub.2 to these ranges, respectively, the concentration of current into an end portion of the cell portion of the device can be prevented upon cut-off of current from an inductive load.