An entangled photon pair generating apparatus has a first definite polarization photon pair generating apparatus for generating a first definite polarization photon pair beam according to an incident pump beam, a wave plate for rotating by 90 degrees a plane of polarization of the first definite polarization photon pair beam emitted from the first definite polarization photon pair generating apparatus, and a second definite polarization photon pair generating apparatus for generating a second definite polarization photon pair beam according to the incident pump beam. They are adjacently placed in order along an optical axis of the incident pump beam.