With respect to a work obtained by joining a semiconductor wafer and a supporting member to each other via a both-faced adhesive sheet having heating separability, the surface of the supporting member is suction-held at a suction stage and is heated, thereby making the adhesive strength of an adhesive layer almost disappear. The device does not require impinging radiation to be mechanically aligned using pinholes, apertures or mechanical slits. The first polyether has an acid number of at least 8, preferably at least 12, and the second polyether has 30-70 wt % of lipophilic parts. The values of infrared energy absorbed by the contaminant on the surface are determined at the first and second wavelengths.