A method and an apparatus for precisely detecting a trench S in a product W to become a thin film solar cell are provided. In the product W, a lower electrode layer 12, in which the trench S is created, and light absorbing layers 13 and 14 are layered on a substrate 11 in this order. The method includes the steps of: detecting infrared rays for imaging, of which the wavelengths are in such a range that can transmit through the light absorbing layers 13 and 14 and which are irradiated from the product W, by means of an infrared ray imaging apparatus 16 that is provided above the light absorbing layers 13 and 14 so that image data for radiation intensity distribution can be taken; and detecting the trench S in the lower electrode layer 12 on the basis of this image data for radiation intensity distribution.
