A signal transmitting device includes: a synchronous-data detecting unit that detects, from parallel data specified by a predetermined format and including video data and audio data, synchronous data for controlling synchronization of the parallel data; an audio extracting unit that stores the audio data in an audio memory; a clock extracting unit that extracts a reference clock from the parallel data; a multiplexing unit that multiplexes the audio data and the synchronous data in a horizontal auxiliary data space of the video data; a control unit that controls, on the basis of the synchronous data and the reference clock, timing of the multiplexing unit for multiplexing the audio data with the video data; and a parallel serial converting unit that converts the video data multiplexed with the audio data by the multiplexing unit into a transmission stream specified by a Level A of a 3G-SDI format.