Constitutively active mutants (CAMs) of G-protein-coupled receptors (GPCRs) cause various kinds of diseases. Rhodopsin, a light absorbing GPCR in animal retinas, has retinal as an endogenous ligand; only very low levels of activation of G-protein can be obtained with the ligand-free opsin. However, the CAM of opsin activates G-protein much more efficiently than the wild type, but the mechanism underlying this remains unclear. The present work revisits the constitutive activity of rhodopsin from the standpoint of conformational dynamics. Single-molecule observation of the M257Y mutant of bovine rhodopsin demonstrated that the switch between active and inactive conformations frequently occurred in M257Y opsin, and frequent generation of the active state results in the population shift toward the active state, which accounts for the constitutive activity of M257Y opsin. Our findings demonstrate that the protein function has a direct connection with the structural dynamics.