PURPOSE: To apply the titled circuit to a frequency range over a video signal band by providing a driving MOS transistor(TR), a load element and a variable resistive element connected in parallel therewith so as to reduce components and increase the gain band width.

CONSTITUTION: A source follower circuit 30 consists of the MOS driving TR31 and the load element 32. An MOS control TR33 is a variable resistive element connected in parallel with said TR31. A DC control voltage is applied from a variable power supply 34 to a gate of said TR33. The gain variable circuit with this constitution controls the gain by utilizing that the gain of the source follower circuit 30 is decided almost definitely by a mutual conductance gm of the driving TR31. Since the number of components is less, the cause to deterioration in the gain band is less and the circuit is applied to a frequency range over the video signal band.