Abstract: A small volume prover is disclosed which is compact and capable of obtaining highly reliable measurement, keeping a highly accurate base volume without being affected by temperature and pressure of fluid to be measured.The prover includes a cylindrical outer housing having a fluid inlet and a fluid outlet spaced apart from each other, a cylindrical measuring conduit having both open ends and first and second sets of fluid ports radially made in a wall thereof and coaxially mounted within the outer housing and an annular wall mounted between the outer housing and the measuring conduit at the position between the measuring conduit open end and the first set of ports thereof to form an upstream annular passage and a downstream annular passage. During proving preparation a piston is restrained by a piston actuator provided in the outlet-side end of the outer housing to permit the fluid to pass through the annular passage.