To provide a method for effectively activating an impurity by using a quick heating method.

A method for manufacturing a thin film semiconductor device comprises a semiconductor thin film forming step of forming a semiconductor thin film 5 having crystallinity in a state in which the film is coupled along a main surface of a substrate 10 to manufacture the device in which a plurality of thin film transistors are distributed on the substrate 10, each thin film transistor has a top gate structure in which a gate electrode 1 is arranged on an upper surface of the film 5 via a gate insulating film 3, an element region isolating step of isolating the film 5 of the continued state to individual element regions by etching, a gate insulating film forming step of forming the film 3 to cover the separated films 5 on the element regions, a gate electrode forming step of forming the gate electrode 1 on the film 3, an impurity implanting step of forming a source region S and a drain region D by field accelerating and emitting an ionized impurity to the film 3 and implanting the impurity of a dose of a range capable of preserving the crystallinity to the film 5, and an impurity activating step of activating the impurity implanted in the film 5 by a quick heating method using a lamp as a heat source.