1. A synaptic semiconductor device comprising: a floating body as a short-term memory means formed of a first conductivity type semiconductor material and electrically isolated from the surroundings; a source and a drain formed of a second conductivity type semiconductor material opposite to the first conductivity type on both sides of the floating body with being separated from each other; a gate formed of a conductive material on a gate insulating layer formed on the floating body; and a long-term memory means formed at one side of the floating body not formed of the source, drain and gate.