An optical transmission apparatus that improves the reliability of system operation by efficiently limiting the line speed of a signal sent from a user with a guard mechanism. An optical branching section makes an optical signal branch in two directions. A through branching line passes an optical signal in its original condition and sends the optical signal as a through optical branch signal. A delay branching line delays an optical signal by a delay amount set and sends the optical signal as a delayed optical branch signal. An optical multiplexing section combines the through optical branch signal and the delayed optical branch signal. A line speed limitation section limits acceptance of the line speed of the optical signal inputted by generating a multiplexed optical signal that meets a pulse mask standard for an optical signal to be passed for the optical signal which is sent at a line speed lower than or equal to a line speed settled under contract with a user and which is to be passed and by setting the delay amount for generating a multiplexed optical signal that does not meet a pulse mask standard for an optical signal acceptance of which is to be refused for the optical signal which is sent at a line speed higher than the line speed settled under contract with the user and acceptance of which is to be refused.