It is an object of the present invention to enhance laser beam transmitting efficiency by accurately controlling an interval between a light emitting (light receiving) element (20) and an optical wave-guide substrate (1) without causing any fluctuation in the interval in a mounting structure of the light transmitting element in which the light emitting (light receiving) element (20) is mounted on the optical wave-guide substrate (1). When the light emitting (light receiving) element is joined to the sub-mount chip (4) and when the sub-mount chip (4) is joined to the optical wave-guide while the element is being directed to the substrate side, the sub-mount chip and the optical wave-guide substrate are joined to each other by the solder bump (6). A post (5) is arranged for regulating an interval between the light emitting (light receiving) element and the optical wave-guide substrate.