An alloying method includes the steps of forming a metal layer on a semiconductor having been transferred to a material having a low thermal conductivity, and alloying an interface between the semiconductor and the metal layer by irradiating the interface with a laser beam having a wavelength absorbable in at least one of the semiconductor and the metal layer. The irradiation energy of the laser beam is set in a range of 20 to 100 mJ/cm.sup.2. The material having a low thermal conductivity is a resin or amorphous silicon. According to the alloying method using laser irradiation, since the entire semiconductor is not heated and only a necessary portion is locally heated, the necessary portion can be readily alloyed to be converted into an ohmic contact without exerting adverse effect on characteristics of the semiconductor device. In the case of previously transferring a semiconductor to a material having a low thermal conductivity, for example, into a resin, and irradiating an interface between the semiconductor and an electrode with a laser beam, the interface between the semiconductor and the electrode can be alloyed at a low temperature, to lower the irradiation energy of the laser beam.