A surface plasmon sensor includes a light guide reflection plate, a surface plasmon resonance layer formed on a first surface of the light guide reflection plate, a light emitting unit having a light source disposed on an end surface of the light guide reflection plate, and a light receiving element. The surface plasmon resonance layer includes a metal layer. The light guide reflection plate includes at least one first reflection surface inclined against the first surface. The light guide reflection plate is configured to transmit light emitted by the light source. The at least one first reflection surface is configured reflect the light to the surface plasmon resonance layer. The metal layer is configured to reflect the light reflected by the at least one first reflection surface. The light receiving element is configured to receive the light reflected by the metal layer.