The invention aims to provide a method for manufacturing semiconductor substrate, and a method for manufacturing semiconductor device. The semiconductor substrate is formed by fixing a monocrystal semiconductor layer on a low heat resistant with a buffer layer there between. A semiconductor substrate is irradiated with accelerated hydrogen ions, thereby forming a damaged region including a large amount of hydrogen. After a single crystal semiconductor substrate and a supporting substrate are bonded to each other, the semiconductor substrate is heated, so that the single crystal semiconductor substrate is separated in the damaged region. A single crystal semiconductor layer which is separated from the single crystal semiconductor substrate is irradiated with a laser beam. The single crystal semiconductor layer is melted by laser beam irradiation, whereby the single crystal semiconductor layer is recrystallized to recover its crystallinity and to planarized a surface of the single crystal semiconductor layer. After the laser beam irradiation, the single crystal semiconductor layer is heated at a temperature at which the single crystal semiconductor layer is not melted, so that the lifetime of the single crystal semiconductor layer is improved.