The present invention is directed to solve a problem that, in a semiconductor device capable of generating a clock signal by coupling a quartz oscillator to an external terminal to which an I/O port is coupled, leak current of the I/O port which is in the inactive state disturbs activation of a clock. The semiconductor device has a first terminal, an amplification circuit coupled to the first terminal, and an output buffer whose output terminal is coupled to the first terminal. The output buffer has first and second transistors of a first conduction type coupled in series via a first node between a first power supply line and an output terminal, and the conduction states of the first and second transistors of the first conduction state are controlled in response to a first control signal which is applied commonly to the gate of each of the first and second transistors.