We propose a deep floating guard ring (DFGR) structure which effectively prevents the curvature breakdown of a central junction in a planar InGaAs/InP avalanche photodiode (APD). In order to investigate the DFGR APD performance, the dark current, photocurrent, and radial dependence of gain were measured and analyzed. In addition, the relation between breakdown voltage and multiplication layer thickness was calculated using a nonlocal history-dependent model. Reliable operation in the device center region was examined by measuring the radial gain. As a result, it has been confirmed that the DFGR is very useful for an APD with a very thin multiplication layer.