Provided is an optical element in which a groove for a sample to make contact can be formed without machining or etching a material for an optical element, and in which the scattering of a light at the groove does not decrease the precision in an optical measurement. The optical element is formed with a light-emitting prism including a light-output face from which a light to be emitted to a sample is outputted, a light-receiving prism including a light-receiving face which receives the light returned from the sample, and a light intensity-reducing part provided between the light-emitting prism and the light-receiving prism. The light-emitting prism and the light-receiving prism are combined to form a recessed part for the sample to make contact, so that the light outputted from the light-output face travels straight in the sample in contact with the recessed part and enters the light-receiving face.