Provided are a resinous reflecting optical element that achieves high mirror surface precision by mitigating the warping effects associated with contraction during resin hardening and suppressing the distortion of a mirror surface that results from resistance to mold release, and a scanning optical device that uses said reflecting optical element. The reflecting optical element is characterized by having a long, tabular substrate (3), a mirror surface section (2) positioned on one surface of the substrate (3), and a hollow portion (4) positioned within the interior of the substrate (3), and is also characterized in that, as a result of configuring so that the hollow portion (4) is longer than the mirror surface section (2), warping and sink marks which form due to contraction during resin hardening are mitigated across the entire mirror surface section (2), and in that the entire mirror surface section (2) protrudes above the surface of the substrate (3), thereby suppressing the increase in resistance to mold release that occurs when a metallic mold is gripped by the molding during resin contraction, and preventing distortion of the mirror surface section (2) that is caused by resistance to mold release.