An electrical machine includes an exhaust fan and an exhaust port on which an optional device is mounted. The optional device includes a duct, an electric fan, and an air flow sensor. Through the duct, an exhaust gas from the exhaust port is guided. The electric fan draws in the exhaust gas flowing through the duct and discharges the exhaust gas into an atmosphere. The air flow sensor detects presence or absence of the exhaust gas flowing from the electrical machine. The air flow sensor includes an air path partitioned from an inside of the duct by a partition wall. The air path includes an exhaust gas and an exit port. The exhaust gas intake port includes an opening facing the exhaust gas. The exit port communicates with the atmosphere at a position outside the duct.