Evaluation of an amount of immobilized sample in a surface plasmon resonance (SPR) immunoassay is provided. To immobilize the sample, a sensor unit has a sensing surface. The sensor unit includes a transparent dielectric medium, and a metal film having the sensing surface. The metal film is connected with the dielectric medium for constituting a metal/dielectric interface. The sensing surface is adapted to sensing reaction of the sample. Then illuminating light reflected by the metal/dielectric interface is received with a photo detector such as a CCD upon applying the illuminating light to the metal/dielectric interface, to detect attenuation of the illuminating light and output a detection signal thereof. The evaluated amount of the sample immobilized on the sensing surface is evaluated according to the detection signal. Before immobilization, the sensor unit is tested for the photo detector to output an initial detection signal, which is considered for the evaluation.