A structure of an engine transmission box is provided. The transmission box includes a transmission box body and a cover arranged on and covering the transmission box body. The transmission box includes at least one airflow entrance opening and an air outlet opening. The cover includes an air inlet port portion, which has an air inlet opening, and a filter member arranged outside of the air inlet opening. An airflow guide hood is arranged to cover the air inlet port portion. The filter member is located between the airflow guide hood and the air inlet port portion. With such an arrangement, a space is guaranteed inside the transmission box, and cleaning or replacement engineering of the filter member is made easy.