To provide a quantum entangled photon pair generating device capable of generating a high-purity quantum entangled photon pair without causing an unnecessary excessive loss or the like by using a simple, readily available optical component.

A polarization maintaining loop optical path having a polarization splitting-combining module as both ends has first and second second-order nonlinear optical mediums, and a half-wave plate located in the middle of the mediums and rotating a polarization plane by 90 degrees. The first second-order nonlinear optical medium produces first SHG light from split excitation light propagating counterclockwise, and generates a correlative photon pair by the SPDC conversion of second SHG light propagating clockwise generated by the second second-order nonlinear optical medium, and the second second-order nonlinear optical medium performs aimed processing thereto. Two pairs of relative photons propagating in a reverse direction are combined by the polarization splitting-combining module, and then signal light and idler light are separately extracted. The loop optical path includes a polarization plane manipulation means that manipulates the polarization plane of one of optical components for combining the photon pair.