We consider downlink resource sharing between multiple cellular users (CUs) and device-to-device (D2D) links, where each D2D link reuses multiple CU channels and each CU channel is shared by multiple D2D links. The design goal is to provide quality-of-service protection for CUs and meanwhile enhance the spectral efficiency of D2D links, which leads to a difficult resource sharing problem. We propose a novel efficient approach to address the downlink multichannel D2D resource sharing problem. Numerical results show that the proposed approach exhibits superiority over existing D2D schemes.