This paper proposes an all-digital process variability monitor based on a shared structure of a buffer ring and a ring oscillator. The proposed circuit monitors the PMOS and NMOS process variabilities independently according to a count number of a single pulse which propagates on the ring during the buffer ring mode, and an oscillation period during the ring oscillator mode. Using this shared-ring structure, we reduce the occupation area about 40% without loss of process variability monitoring properties compared with the conventional circuit. The proposed shared-ring circuit has been fabricated in 65nm CMOS process and the measurement results with two different wafer lots show the feasibility of the proposed process variability monitoring scheme.