Methods for manufacturing a semiconductor substrate and a semiconductor device by which a high-performance semiconductor element can be formed are provided. A single crystal semiconductor substrate including an embrittlement layer and a base substrate are bonded to each other with an insulating layer interposed therebetween, and the single crystal semiconductor substrate is separated along the embrittlement layer by heat treatment to fix a single crystal semiconductor layer over the base substrate. Next, a plurality of regions of a monitor substrate are irradiated with laser light under conditions of different energy densities, and carbon concentration distribution and hydrogen concentration distribution in a depth direction of each region of the single crystal semiconductor layer which has been irradiated with the laser light is measured. Optimal irradiation intensity of laser light is irradiation intensity with which a local maximum of the carbon concentration and a shoulder peak of the hydrogen concentration are observed. A single crystal semiconductor layer is irradiated with optimal laser light at energy density detected by using the monitor substrate, whereby a semiconductor substrate is manufactured.