A ventilation fan includes a body unit, which has a fan disposed in a body case and a fan controller operable to control an air volume of the fan. The ventilation fan also includes a sensor module having an indoor environment detecting sensor and a module controller operable to control a detecting operation of the indoor environment detecting sensor. The body unit has a module mount on which the sensor module is mounted and another sensor module can be mounted in place of the sensor module. The module mount is connected to the fan controller. The module controller outputs detection information obtained based on a detection result of the indoor environment detecting sensor to the fan controller, which in turn controls the air volume of the fan based on the detection information inputted thereto.