A target substance is detected by a sandwich immunoassay using fine particle (A) having bound to it a fluorescer and an antibody reacting specifically with the target substance, and a fine particle (B) having bound to it a quencher and an antibody reacting specifically with the target substance, through a different antigenic determinant. Also disclosed is a competitive immunoassay having a fine particle (C) bound to it a fluorescer or a quencher, and an antibody reacting specifically with the target substance, a bound product (D) composed of the remainder of the fluorescer and the quencher, or a known amount of the target substance. Binding of the fluorescer and the quencher to the fine particle (A), (B) or (C) is affected so that the fluorescer and the quencher are covalently bound to a substance adsorbed on the fine particle. The sandwich immunoassay is advantageously conducted using a kit containing (i) the fluorescer- and antibody-bound fine particle (A) and (ii) the quencher- and antibody-bound fine particle (B). The competitive immunoassay is advantageously conducted by using a kit containing (i) the fluorescer- or quencher-and-the antibody-bound fine particle (C), and (ii) the bound product (D) of the quencher or fluorescer and the target substance.