A driving force transmission device superior in reliability with suppressed rotation fluctuation. The driving force transmission device has gearing reception portions, three arm portions and a press member. The gearing reception portions are formed in the inner circumference of a rotor. The arm portions extend from a shaft radially. The arm portions can be elastically deformed. The arm portions have engagement tooth portions for transmitting a driving force of the shaft to the rotor. The press member presses the rotor to bend the arm portions in the direction where the rotor is pressed. In that state, the gearing reception portions gear with the engagement tooth portions.