A flexible device is manufactured at low temperatures. A second substrate is bonded to a first substrate by a first adhesive layer. A first insulating layer, a transistor, and a second insulating layer are formed over the second substrate. Then, the first substrate and the second substrate are separated from each other while being heated at a first temperature. The heat resistant temperatures of the first substrate, the second substrate, and the first adhesive layer are a second temperature, a third temperature, and a fourth temperature, respectively. Each of the first insulating layer, the second insulating layer, and the transistor is formed at a temperature higher than or equal to room temperature and lower than the fourth temperature. The third temperature is higher than the fourth temperature and lower than the second temperature. The first temperature is higher than the fourth temperature and lower than the third temperature.