Provided is an immunochromatographic diagnostic kit which renders prompt diagnosis possible, has high analysis sensitivity, and gives test results with excellent reproducibility. The immunochromatographic diagnostic kit includes: a conjugate pad that contains a conjugate containing chromogenic particles which have an average particle diameter of 100-1,000 nm and attain a color intensity of 1.0-10.0, 10-90 wt % of the particles being derived from cellulose and 90-10 wt % of the particles being derived from a colorant; and a sample pad for an in vitro diagnostic reagent, the sample pad being constituted of nonwoven fabric that has a basis weight of 10-150 g/m2 and a thickness of 0.07-1.00 mm and comprises regenerated cellulosic fibers.