It is an object of the present invention to provide a method for detecting a target substance in a specimen with the use of fine particles, whereby the target substance can be readily detected with the exclusive use of a single type of probe and the detection limit is improved. The present invention provides a method for detecting a target substance in a specimen which comprises the steps of: allowing a complex of a fine particle and a probe to come into contact with a specimen; and detecting changes in physical properties of the fine particle that are caused by desorption of the probe from the fine particle due to interaction between the target substance in the specimen and the probe.