The present invention relates to a reagent for biomaterials assay of (fluorescent dye).sub.n --avidin--a compound having a plurality of reactive groups--a biomaterial, or a reagent in which avidin is bound to a compound having a plurality of reactive groups through biotin in the above, a method for preparing it, a plastic fiber optics necessary for utilizing this, an assaying device having a sensing portion utilizing this fiber optics and an assaying method of biomaterials by use of this reagent and device. The assay method can be used for immunoassay of biomaterials such as antigens, enzymes contained in extremely minute amounts in blood or body fluid in medical diagnosis, and improves detectable sensitivity to a great extent.