Many of the principles of an oxide semiconductor are still unclear and therefore there is no established method for evaluating an oxide semiconductor. Thus, an object is to provide a novel method for evaluating an oxide semiconductor. Carrier density is evaluated, and hydrogen concentration is also evaluated. Specifically, a MOS capacitor (a diode or a triode) is manufactured, and the C-V characteristics of the MOS capacitor are obtained. Then, the carrier density is estimated from the C-V characteristics obtained.