The outer surface of a wide-gap semiconductor device is coated with a synthetic polymer compound containing one or more silicon-containing polymer having a bridged structure formed by a siloxane (Si—O—Si bond structure). The synthetic polymer compound may include, for example, a silicon-containing polymer which has one or more reactive groups (A′) selected from Si—R1, Si—O—R2 and Si—R3—OCOC(R4)═CH2, has a bridged structure formed by an Si—O—Si bond in one or more locations, and contains components having weight average molecular weights of not more than 1000 in an amount of 20% or less by weight.