The present invention intends to provide an immunochromatographic test piece that makes it possible to achieve both highly sensitive detection of a substance to be detected and a simple test piece structure, which are usually difficult to be made compatible with each other. The immunochromatographic test piece is an immunochromatographic test piece comprising a membrane on which a capture substance being a ligand that bonds to a substance to be detected is immobilized, wherein insoluble carrier particles to which a ligand that bonds to the substance to be detected is bound are used and accumulated by being captured with the capture substance immobilized on the membrane, the membrane is irradiated with light to detect light emitted from a portion where the insoluble carrier particles are accumulated or light emitted from a portion surrounding and other than the portion where the insoluble carrier particles are accumulated, thereby measuring the substance to be detected, and a light-reflecting material is provided on a side of the membrane opposite to a side irradiated with light.