An amorphous semiconductor layer is formed over a first single crystal semiconductor layer provided over a glass substrate or a plastic substrate with an insulating layer therebetween. The amorphous semiconductor layer is formed by a CVD method at a deposition temperature of higher than or equal to 100° C. and lower than or equal to 275° C. with use of a silane-based gas not diluted. Heat treatment is performed so that the amorphous semiconductor layer solid-phase epitaxially grows. In such a manner, an SOI substrate including a thick single crystal semiconductor layer is manufactured.