Provided are a slow wave circuit and a traveling wave tube suitable for an increase in fineness with regard to processing beam holes, and suitable for higher frequencies. A slow wave circuit (10) includes a meandering waveguide (1) and a beam hole (2) that pierces the meandering waveguide (1), and the cross-section of the beam hole (2) in the direction orthogonal to the long direction is in the shape of a polygon having a larger number of sides than a quadrilateral.
