To test electrical characteristics of a Thin Film Transistor (TFT) with a source or drain terminal left open and exposed, using a non-contact current source and protecting the TFTs from adverse effects, such as contamination, destruction, and the like. A tester 100 is provided to test a TFT array substrate 14, the tester including ion flow supply devices 16 and 18 for supplying an ion flow onto the surface of a substrate 14. Thereon, an array 12 of TFTs is formed, each TFT being connected to an electrode having a source or a drain left open and exposed; a control circuit 24 for supplying an operating voltage to a gate electrode of the TFT to be tested in the array; and a measurement circuit 24 for measuring an operating current via the testing TFT source or drain that remain in a non open state.