Abstract: An I/O circuit is formed by combining plural types of standard cells contained in a cell library. For example, the standard cell includes a first element forming region having, formed therein, protection target elements each having a gate electrically connected to an external terminal, a second element forming region arranged in immediate proximity to the external terminal and having first protection elements formed therein, and a third element forming region arranged between the first and second element forming regions and having transistors formed therein. The transistors each have a drain connected to gates of the protection target elements and a source, gate, and back gate all connected to a power supply or ground terminal, thus functioning as a second protection element.