The field-angle-resolved specific heat C(T,H,phi) of the f-electron superconductor CeRu2 (Tc=6.3 K) has been measured at low temperatures down to 90 mK on two single crystals of slightly different qualities. We reveal that the C(phi) oscillation in a
rotating magnetic field, originating from the gap anisotropy, diminishes at low temperatures below the characteristic field H*, as expected for an anisotropic gap without nodes. We also observe the suppression of H* by decreasing the gap anisotropy ratio $Delta_{rm min}/Delta_{rm max}$, a behavior that has been predicted from a microscopic theory for anisotropic s-wave superconductors. The present technique is established as a powerful tool for investigating minimum-gap structures as well as nodal structures.
