To provide a bidirectional prover capable of calculating an instrumental error highly accurately, and a bidirectional prover capable of simplifying a device configuration.

Arrangement of one of a pair of pressure detection means 18, 20 is set on a pipe part 14 between a valve and a pipe connecting a pipe side large diameter pipe part 6 of a prover pipe 2 to a four-way valve 13, and arrangement of the other of the pair of pressure detection means 18, 20 is set on a pipe part 15 between the valve and a pipe connecting a pipe side large diameter pipe part 7 of the prover pipe 2 to the four-way valve 13. Additionally, an output signal from the pair of pressure detection means 18, 20 is also used as a signal for determining a flow direction of fluid in the prover pipe 2.