A height of a signal transmission device is decreased as low as possible as maintaining or improving a cooling performance for the communication module. Ina signal transmission device provided with a communication module provided on a substrate and a cooling mechanism for cooling the communication module, the cooling mechanism includes: a heat-transfer plate including a first region which overlaps with bottom surfaces of a plurality of the communication modules and is thermally connected to the bottom surfaces and a second region which does not overlap with the bottom surfaces of the communication modules; and a heat-release fin provided in the second region of the heat-transfer plate.