A phototransistor includes a first emitter region, a first base region having at least a portion exposed to a light-receiving side, and a first collector region in this order from the light-receiving side in a depth direction. The first collector region includes a second collector region and a third collector region that is in contact with a downstream side of the second collector region in the depth direction and has a resistance lower than that of the second collector region. The phototransistor further includes a first region that is spaced away from the first base region at an outer side of the first base region on a light-receiving side surface thereof, the first region having a conductivity type opposite to that of the first collector region.