To provide a semiconductor device which contains an I/O buffer or an output buffer which can control a switching speed of a buffer transistor to be an appropriate value even when process conditions and/or temperatures, etc., change.

A semiconductor device has an output buffer or an I/O buffer (28, 29) which contains a buffer transistor, and control circuits (21p, 21n, 22, 23) which change a size of the buffer transistor. The control circuit changes the size of the buffer transistor based on a switching speed of the buffer transistor or a detection transistor which changes corresponding to process conditions and/or temperatures.