Provided is a surface plasmon resonance sensor device having excellent measurement accuracy. The device is equipped with an optical fiber 2,32 for introducing a light incident from one end thereof to the other end, a core exposed portion 3,33a formed to expose core 5,33 of the optical fiber 2,32, and a thin metal film 4,35 formed on the core exposed portion 3,33a and capable of exciting surface plasmon. The core exposed portion 3 is a first microsphere 3 disposed at the other end portion of the optical fiber 2 and the thin metal film 4 is formed on the surface of the microsphere 3. The thin metal film may be a first fine-metal-particle thin film 25 in which fine metal particles 24 have been distributed uniformly. The core exposed portion 33a is disposed in the midway of the optical fiber 32 and the thin metal film 35 is formed to cover therewith the entire surface of the core exposed portion 33a. The core exposed portion 33a is equipped with a second microsphere 36 formed integral with the core 33 and the thin metal film 35 is formed to cover the microsphere 36. The thin metal film 35 may be a second fine-metal-particle thin film 38 in which fine metal particles 37 have been distributed uniformly.