To provide a masking reagent capable of simply carrying out separate coating between the coated part and the non-coated part at a low cost.

The masking reagent 10 includes a thinly long plate-like masking reagent arranged along a boundary between a coated surface 31 and an non-coated surface 32 and to the non-coated surface 32 side during spray coating, a parallelogram in cross section orthogonal to a longitudinal direction, and an acute angle of the parallelogram of 10 to 60 degree. It is preferable that a pressure sensitive adhesive layer 13 is formed on a face abutted at least to the non-coated surface 32 of the masking reagent 10.