According to one embodiment, a manufacturing method of thin film transistor includes forming an oxide semiconductor layer on a first insulating film, forming a first conductive layer formed of molybdenum or a molybdenum alloy on the oxide semiconductor layer, forming a second conductive layer on the first conductive layer, forming a resist mask on the second conductive layer, and forming a first conductive portion and a second conductive portion by performing dry etching of the second conductive layer using the resist mask.