Substrate for oxide semiconductor thin film transistor

The present invention provides: a substrate for an oxide semiconductor thin film transistor having a high mobility and a threshold voltage within a predetermined range; and a flexible semiconductor device that is provided with the substrate. In the present invention, in the cases of using a base material (10) as a substrate for an oxide semiconductor thin film transistor, said base material having a metal base portion (11) and a porous layer (12), a protection insulating layer (20), which has a silicon compound as a main component, contains hydrogen, and has insulating characteristics, is formed on the porous layer, said protection insulating layer (20) having a hydrogen concentration of 3.5×10 21 -3.5×10 22 atoms/cm 3 , and a thickness of 100-2,000 nm. A thin film transistor is provided on the protection insulating layer (20), said thin film transistor including an oxide semiconductor in an active layer.