In a phototransistor which comprises an emitter, a collector and a base, base portions 20 and 21 are made unequal in impurity density, by which minority carriers of optically excited carriers are stored in the high impurity density regions 20 of the base and majority carriers are permitted to easily pass through the low impurity sensity regions 21 of the base, and voltages of the high and low impurity density regions are coupled together. A very high-sensitivity and high-speed operation can be achieved.