A method for detecting pork in heated food by immunochromatographic detection with high performance and high sensitivity without causing non-specific reaction. The method provides a convenient and high-accuracy detection method using a polyclonal antibody. When a target to be detected in a sample, pork-derived protein in heat-processed food, is detected by immunochromatography, a polyclonal antibody specifically recognizing a protein of approximately 23 kD (molecular weight: 23000) contained in heat-treated pork is used as at least one or both detection antibodies.