Disclosed is a technique for suppressing fluctuation of device characteristics in thin film transistors using an oxide semiconductor film as a channel layer. In a thin film transistor using an oxide semiconductor film as a channel layer (4), said channel layer (4) is configured from an oxide semiconductor having as main materials a zinc oxide and tin oxide with introduced group IV elements or group V elements, wherein the ratio (A/B) of the impurity concentration (A) of the group IV elements or group V elements contained in the channel layer (4) and the impurity concentration (B) of the group III elements contained in the channel layer (4) satisfies A/B≦1.0, and ideally A/B≦0.3.