Thermal connection between a plurality of communication modules and a heatsink placed on these communication modules is securely achieved and maintained. In a signal transmission device in which a common heatsink is arranged on a plurality of communication modules equipped on a board, the signal transmission device has a coil spring provided between the board and the communication modules, and the communication modules are biased toward the heatsink by the coil spring so that an upper surface of the communication module is pressed against a bottom surface of the heatsink.