A package for superconducting devices, enabling signal transmission at a small propagation delay between superconducting devices provided within a heat insulating vacuum vessel, and semiconductor devices provided outside the heat insulating vacuum vessel. The package comprises a heat insulating vacuum vessel capable of maintaining the interior thereof at a low temperature, a substrate having mounted thereon superconducting devices and disposed within the heat insulating vacuum vessel, another substrate having mounted thereon semiconductor devices and disposed outside the heat insulating vacuum vessel, and film cables electrically connecting the respective circuit patterns of these substrates. Since the thickness of the film cables are very small, the rate of influx of heat that flows into the heat insulating vacuum vessel can be limited to a low value.