An outer casing for a gas sensor includes a cap, a mount base, and a heat shielding board. The cap, the mount base, and the heat shielding board are made of gas permeable ceramics. A target gas flows inside the gas sensor with the above outer casing from all directions, and makes the gas sensor highly responsive to the target gas. The outer casing also contributes to make temperature and humidity inside the gas sensor uniform in a short time, thereby stabilizing an output voltage of the gas sensor.