To provide a straightening device that is large in effective sectional area and has high straightening performance, the present invention provides a straightening device (10) arranged in a fluid passage (26) for making fluid pass through it that is provided with: €¢ a main body (11) having an inflow port (12) arranged in the fluid passage (26) and for flowing the fluid into it, an outflow port (13) for flowing the fluid out of it, and a communication passage (14) for communicating the inflow port (12) with the outflow port (13); and €¢ multiple projections (15) that are arranged in a protruding manner toward a central part of the communication passage (14) from an inner peripheral part and extend along the communication passage (14), in which the projections are each configured to have a narrower width in the central part than that in an inner peripheral part of the communication passage (14) when seen from a flow direction of the fluid.