PURPOSE: To discriminate dental caries activity simply and in a short time by wetting spittle specimen with a test piece for discriminating a microorganism of causing dental caries in spittle, sandwiching the test piece in between films having an adhesive layer, sealing, culturing the microorganism and then measuring change in color tone of the test piece.

CONSTITUTION: A test piece comprising a small paper piece 1 prepared by immersing a filter paper piece in a reagent composition which comprises an aqueous solution of resazurin and sucrose and discriminates the amount of microorganism of causing dental caries in spittle and drying at 70°C for 90 minutes is wetted with a fixed amount of spittle specimen, the wetted material is covered with a covering transparent plastic film 2, placed side by side with a supporting film 3, stuck to rectangular board 4, put in a packing sash made of a transparent plastic, made into a sealed state, maintained at room temperature for a given time, the dental caries causing microorganism is cultured and change in color tone of the test piece is measured to discriminate dental caries activity in a dry method.