A sensor unit is for use in a surface plasmon resonance (SPR) assay apparatus having an assay stage. A total reflection prism is supported on a stage surface of the assay stage, and has a sensing surface positioned on an upper surface thereof. The sensing surface receives illuminating light applied thereto to reflect the illuminating light. The assay apparatus receives the illuminating light reflected by the sensing surface, for measuring reaction of a sample. Two engageable ridges are disposed on first and second lateral faces of the prism which are so positioned that the sensing surface is disposed between, and keep the prism positioned on the stage surface by engagement with a retention mechanism of the assay apparatus. Furthermore, a grip portion is formed at a first end of the prism, and adapted to holding of the prism.