To provide a coupling optical system capable of suitably performing recording and/or reproduction of information by suppressing erroneous detection of an optical detector, to provide an optical element and to provide an optical pickup device.

When an intermediate plane IP of a first lens CUL is inclined, a luminous flux made incident in the intermediate plane IP is emitted in a state of a divergent luminous flux from an optical disk side optical surface S2. Thereby, incidence of the luminous flux passing through the intermediate plane IP in an effective diameter of an objective lens OL is suppressed and erroneous detection can be suppressed. The intermediate plane IP may be not only a tapered surface but also a spherical surface or an aspherical surface formed by extending a light source side optical surface S1.