It is intended to enable the short-time and accurate determination of the concentration of a substance that is measured by a gelation reaction such as an endotoxin or beta-D glucan. In an apparatus for measuring a target substance in a sample via a gelation reaction, a sample cell (13) which contains a solution containing the sample containing the substance to be measured and a reagent causing gelation is irradiated with illuminating light from a light-emitting diode (14). The solution in the sample cell (13) is stirred with a stirrer (25) to form minor and uniform gel particles and then allowed to pass through the illuminating light. The transmitted light from the gel particles formed in the sample cell (13) is detected with a diode (22). Based on a time lag until the amount of the detected transmitted light attains a certain level or below by a computer (21) on the basis of the transmitted light detection output thus obtained, the concentration of the above-described substance in theabove-described solution is determined.