PROBLEM TO BE SOLVED: To provide a light transmission device and an optical element, which are relatively inexpensive and excellent in assembly easiness and can be miniaturized.SOLUTION: A prism comprises: a first plane which contacts the end face of an optical fiber a second plane for reflecting an optical beam emanating from the end face of the optical fiber and a third plane for emanating the optical beam reflected on the second face. If the angle of the second plane with respect to the axis of the optical fiber is designated as θ1 if the angle of the center of the optical beam having emanated from the end face of the optical fiber with respect to the axis of the optical fiber is designated as θ2, if the inclination angle of the optical fiber end face with respect to the direction perpendicular to the axis of the optical fiber is designated as θ3, if the numerical aperture owned by the optical beam having emanated from the end face of the optical fiber into the prism is designated as θNA, if the core portion of the optical fiber has a refractive index nf, and if the prism has a refractive index ng, and if a medium to be contacted by the second face of the prism has a refractive index na, a relation of ng>na, and the following relations hold: ng>na/{sin(90°-θNA-θ2 + θ1)}, and 0≤sin-1 (nf×sinθ3/ng)-θ3<90.COPYRIGHT: (C)2014,JPO&INPIT【課題】比較的安価であり、組立容易性に優れ、しかも小型化を図れる光伝達装置及び光学素子を提供する。【解決手段】プリズムは、光ファイバの端面と接する第1面と、光ファイバの端面から出射した光線を反射する第2面と、第2面で反射した光線を出射する第3面とを有し、光ファイバの軸線に対する第2面の角度をθ1とし、光ファイバの軸線に対する光ファイバの端面から出射する光線の中心とのなす角度をθ2とし、光ファイバの軸線直交方向に対する光ファイバ端面の傾き角をθ3とし、光ファイバの端面からプリズム内に出射した光線が持つ開口数をθNAとし、光ファイバのコア部の屈折率をnfとし、プリズムの屈折率をngとし、プリズムの第2面が接する媒体の屈折率naとしたときに、ng>naを満たすとともに、以下の式が成立する。ng>na/{sin(90°-θNA-θ2+θ1)}、0≦sin-1(nf×sinθ3/ng)-θ3<90【選択図】図1
