To detect presence of an organic matter solderable protective film (OSP) on a substrate by placing a liquid droplet on the surface of a copper substrate, for example, and measuring a contact angle defined by the wall of liquid droplet and the surface of substrate.

When a liquid droplet D for test is placed on the surface S of a metal substrate (e.g. copper), an image is formed by the liquid droplet. Contact angles A, B, defined by the wall of liquid droplet D and the surface S of metal substrate, are then measured and compared with a known contact angle defined by an OSP selected from a group of imidazole and a bare surface S. Since the contact angles A, B lack symmetry, measurements are taken by number of times sufficient for obtaining a mean measurement appropriate statistically for the counterpart contact angle. Since the uncertainty increases as the interval between preparation of sample and test increases, the time between preparation and test is set short, preferably within 5-20min.