A circuit unit includes a circuit board, a cover being configured to house the circuit board inside, a first airflow inlet formed in a first surface of the cover, and a first duct formed in the first surface wherein the first duct is in communication with the first airflow inlet, and a sliding mechanism that supports the circuit unit such that the circuit unit is movable, the sliding mechanism includes a fixed frame, a second airflow inlet formed in the fixed frame, and a second duct that is formed in the fixed frame and in communication with the second airflow inlet, and, when the circuit unit is located at the first position, the first duct and the second duct are engaged with each other such that a third duct is formed to cause the first airflow inlet and the second airflow inlet communicate each other.