A method for manufacturing a semiconductor substrate, the semiconductor substrate including: a substrate; an initial layer provided on the substrate; a high-resistance layer provided on the initial layer which is composed of a nitride-based semiconductor and contains carbon; and a channel layer provided on the high-resistance layer which is composed of a nitride-based semiconductor, and at a step of forming the high-resistance layer, a gradient is given to a preset temperature at which the semiconductor substrate is heated, and the high-resistance layer is formed such that the preset temperature at the start of formation of the high-resistance layer is different from the preset temperature at the end of formation of the high-resistance layer. It is possible to provide the method for manufacturing a semiconductor substrate, which can reduce a concentration gradient of carbon concentration in the high-resistance layer and also provide a desired value for the carbon concentration.