In a manufacturing process of a semiconductor device formed using a thin film transistor, an object is to provide a technique by which the number of photomasks can be reduced, manufacturing cost can be reduced, and improvement in productivity and reliability can be achieved. A main point is that a film forming a channel protective layer is formed over an oxide semiconductor layer having a light-transmitting property, a positive photoresist is formed over the film forming a channel protective layer, and a channel protective layer is selectively formed over a channel formation region in the oxide semiconductor layer by using a back surface light exposure method.