In a prior art, there has been a method in which a power supply line of an output buffer and that of a control circuit are independently provided so that the power supply noise occurring in the control circuit will not affect the output buffer. However, this method has had the problems that it increases both the number of power supply/grounding pins and power feed line inductance. The present invention provides a technique which, without causing the above two problems, i.e., (1) increased number of power supply/grounding pins and (2) increased power feed line inductance, prevents the noise causing a problem in a control circuit, from becoming routed around and induced into an output buffer. More specifically, the above can be realized by using either of two methods: (A) providing an on-chip bypass capacitor for the control circuit and isolating a power feed route of the control circuit from that of the output buffer in an AC-like manner, or (B) designing electrical parameters (inserting resistors) such that the oscillation mode of any electrical parameter noise induced into the power feed routes will change to overdamping.