Viscosity of a test body solution spotted onto a test piece is detected. The test piece has an insoluble support, through which the test body solution is developed, and a test region formed on the insoluble support, the test region reacting with an analyte in the test body solution and undergoing coloration. A washing liquid is fed to the test region and its neighboring regions at the time between a stage, at which the test body solution has been developed through the test region, and a stage, at which a liquid for amplifying the coloration state of the test region is fed to the test piece, such that the quantity of the washing liquid fed to the test region and its neighboring regions is set to be large as the detected viscosity becomes high.