Highly sensitive and selective detection of methamphetamine (MA) was realized by using an immunosensor based on surface-plasmon-resonance (SPR) phenomena. A methamphetamine-bovine serum albumin (MA-BSA) conjugate was prepared as an antigen. It was immobilized on the Au thin film of sensor chip by physical adsorption. The incident angle of the MA-BSA immobilized sensor increased with increasing concentration of anti-MA-BSA antibody up to ca. 20 mu g/ml (ppm) and then almost saturated above this concentration. The addition of MA into the antibody solution (20 ppm) was found to decrease the incident angle shift sharply because of the inhibition effect of MA. The present sensor was confirmed to detect MA very sensitively in the concentration range of 0.1-1000 ng/ml (ppb). Moreover, it was found that the sensor could detect MA in the PBS solution containing 1% human urine without any significant changes in incident angle shift due to nonspecific adsorption. Furthermore, it was confirmed that the interference effect of methylephedrine on the MA sensitivity of the sensor was fairly small. From the correlation between incident angle shift and MA concentration, the affinity constants of anti-MA antibody to MA (in liquid media) and MA-BSA (immobilized) ere evaluated by assuming Langmuir isotherm and equilibrium state of immunoreaction.