Using high-speed single-molecule tracking optical techniques, we measure single lipid diffusion in the cell plasma membrane at 10,000 frames per second with nanometer spatial precision. We observe highly heterogeneous nanoscopic diffusion both for the saturated and non-saturated phospholipids. The restricted diffusion shows dependency on the cholesterol concentration in the membrane, indicating that the cholesterol-mediated phase separation at the nanoscale acts as a regulation mechanism of membrane dynamics.