Provided are a latex agglutination reagent containing polyoxazoline that is preferable for use in a latex agglutination reaction and has a high sensitization action of sensitivity while a non-specific reaction can be suppressed, and a method for measuring a target substance in a specimen by a latex agglutination method. A latex agglutination reagent used in a method for measuring a target substance in a specimen by a latex agglutination method using an insoluble carrier that carries an antibody or an antigen contains polyoxazoline having a repeating unit represented by the following General Formula (in Formula (1), R represents a hydrogen atom or a methyl group, an ethyl group, or a propyl group, and n is an integer of 100 or greater).