A light guide mechanism for an illuminance sensor includes: a lighting window configured to pass through an external light; the illuminance sensor configured to detect an illuminance of the light passed through the lighting window; and a light-blocking part provided between the lighting window and the illuminance sensor and configured to block a light other than the light passed through the lighting window. The light-blocking part has an inner surface inclined with respect to a line perpendicular to a light-receiving surface of the illuminance sensor. A direction of inclination of the inner surface is such that the inner surface faces the light-receiving surface of the illuminance sensor.