There are provided a substrate for mounting a device and a package for housing the device employing the same in which a power semiconductor device can be readily set for a temperature suitable for operation and can thus function in a proper fashion.The substrate for mounting the device includes a support body having, on one main surface of the support body, a device mounting portion for mounting a power semiconductor device, the support body having a plurality of columnar parts that are spaced apart in a thickness direction with respect to the device mounting portion and are arranged apart from each other; and a heat accumulating region which is disposed between the columnar parts and is lower in thermal conductivity than the support body.