[Problems] An object of the present invention is to provide a novel method which can expand the measurement range in an immunoassay method.[Means for Solution] The immunoassay method according to the present invention comprises allowing a polyoxyethylene alkyl ether sulfate and a polyoxyethylene sorbitan fatty acid ester to coexist in a reaction system.[Effects] According to the method of the present invention, variation (standard deviation) in blank measurement values in particular can be reduced, so that measurement values are stabilized. By this, a significant difference can be obtained in measurement values even in a low-concentration range where conventional immunoassay methods have difficulty in performing measurements; therefore, the measurement accuracy in such low-concentration range can be improved.