Provided is a method by which the quantity of objects to be detected can be quickly determined with high sensitivity. The method of determining the quantity of objects to be detected in a specimen comprises: a mixing step for preparing a mixture by mixing the specimen and carrier particles that carry stimulus-responsive substances and a first affinity substance for the objects to be detected; a measurement step for placing the mixture under the condition that the stimulus-responsive substances aggregate, and measuring the particle diameter of a suspended solid in the mixture; and a determination step for determining, on the basis of the particle diameter, the quantity of the objects to be detected.