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A bathtub vortex in a cylindrical tank rotating at a constant angular velocity Ω is studied by a laboratory experiment, a numerical experiment and a boundary layer theory. The laboratory and numerical experi-ments show that two regimes of vortices in the steady-state can occur depending on Ω and the volume flux Q: When Q is large and Ω is small, a potential vortex in which angular momentum is constant outside the vortex core is formed (Regime I). When Q is small or Ω is large, however, a vortex in which the angular momentum decreases with decreasing radius is generated (Regime II). The boundary layer theory shows that the vortex regimes strongly depend on the theoretical radial flux through the bottom boundary layer under a potential vortex: When the ratio of Q to the theoretical boundary layer radial flux Qb (scaled by 2πR2(νΩ)1/2) at the outer rim of the vortex core is larger than a critical value (of order 1), the radial flow in the interior exists at all radii and Regime I is realized. When the ratio is less than the critical value, the radial flow in the interior nearly vanishes inside a critical radius and almost all of the radial flux occurs only in the boundary layer, resulting in Regime II in which the angular momentum is not constant with ra