This composite substrate has a single-crystal semiconductor thin film (13) provided to at least the front surface of an inorganic insulating sintered-body substrate (11) having a thermal conductivity of at least 5 W/m·K and a volume resistivity of at least 1×108 Ω·cm. The composite substrate also has, provided between the inorganic insulating sintered-body substrate (11) and the single-crystal semiconductor thin film (13), a silicon coating layer (12) comprising polycrystalline silicon or amorphous silicon.As a result of the present invention, metal impurity contamination from the sintered body can be inhibited, even in a composite substrate in which a single-crystal silicon thin film is provided upon an inexpensive ceramic sintered body which is opaque with respect to visible light, which exhibits an excellent thermal conductivity, and which further exhibits little loss at a high frequency range, and characteristics can be improved.