To provide a detector allowing anyone to detect easily and surely a fruit tree pathogen in an outdoor field such as a field.

The first and second antibodies having specific bondability to the fruit tree pathogen are prepared, a dropping part 11 for dropping a sample liquid, a labeled section 12 for holding the first antibody labeled with colored particle flowably under a wet state wetted by the sample liquid, and a detecting section 14 with the second antibody brought into a solid phase, are adjacently laid sequentially from an upstream toward a downstream along a flow direction of the sample liquid, and a collation section is provided in a position different from the detecting section. The first and second antibodies are anti-fruit-tree-pathogen monoclonal antibodies by obtaining antibody production hybridoma cells derived from an animal immunized by the fruit tree pathogen, followed to be cultured/refined.