A sensor unit for assay in utilizing surface plasmon resonance (SPR) includes a dielectric prism. Metal film has a light entrance surface fitted on the prism, and a sensing surface reverse to the light entrance surface. The sensing surface is adapted to causing interaction between ligand of ligand liquid and analyte liquid. An optical assay unit includes a light source, optical system and photo detector. A flow channel includes the sensing surface positioned inside. In an immobilizing stage, the ligand is immobilized on the sensing surface by introducing the ligand liquid to the flow channel. Evaporation retardant is introduced to the flow channel, and prevents drying of the ligand. The evaporation retardant is removed from the flow channel after transfer of the sensor unit from the immobilizing stage to an assay stage. The analyte liquid is introduced to the flow channel, to assay the interaction.