PURPOSE: To enable detection simply and at a high sensitivity by forming a magnetic reaction product from a material to be measured and a magnetic specific reaction material to trap the product on a chromatographic medium by a magnetic force.

CONSTITUTION: A medium 6 containing an antigen as substance to be measured is placed into a container 1 and when a magnetic antibody reacting specifically to the antigen and an antibody which labeled reacting specifically to the antigen at a part differing from the magnetic antibody are added to the medium, a reaction product magnetic and labeled too is generated by an antigen/antibody reaction. As a sample 6 containing the reaction product moves in capillarity toward a water absorbing body 5 on a chromatographic medium 2, the reaction product is trapped on the medium 2 by a magnetic force at a position where a magnet 4 is provided while a development by a label occurs at the position. Thus, the presence and quantity of the antigen are detected in the sample 6 by checking the presence, intensity, color tone and the like of the development of the label at the position of the magnet 4 on the medium 2.