An object is to provide a structure for forming a circuit for which high-speed operation and low-voltage operation are required and a circuit for which sufficient reliability is required at the time of high voltage application in a circuit group provided over one substrate in a semiconductor device, and a manufacturing method thereof. A semiconductor device is provided with a plurality of kinds of transistors which include single-crystal semiconductor layers with different thicknesses, which are separated from a single-crystal semiconductor substrate and bonded, over one substrate. The single-crystal semiconductor layer of a transistor for which high-speed operation is required is formed thinner than that of a transistor for which high resistance to a voltage is required, so that the thickness of the single-crystal semiconductor layer is made to be thin.