An optional device is for an electrical machine. The optional device is mountable to the electrical machine without electrical connection to the electrical machine. The optional device includes a duct, an air flow sensor, an air purifier, and a controller. Through the duct, an air flow containing an exhaust gas from the electrical machine is guided. The air flow sensor is configured to detect presence or absence of the exhaust gas from the electrical machine. The air purifier is configured to purify the exhaust gas and discharge the purified exhaust gas into an atmosphere. The controller is configured to control an operation of the air purifier based on an output from the air flow sensor.