Hagen-Poiseuille and thermal transpiration flows of a highly rarefied gas through a circular pipe are investigated on the basis of the linearized Boltzmann equation for hard-sphere molecules with the diffuse reflection boundary condition. A numerical computation based on an iterative approximation method with an explicit convergence estimate is performed to construct a database of the mass fluxes for an arbitrary value of large Knudsen numbers. The singular behavior of the velocity distribution functions in that rarefied regime is also clarified.