Expressions of the velocity potential of a source in a rectangular canal are obtained, in a more refined manner than the previous paper, based on the expressions of Dirac's δ function. This method of solution may in general be possible when the solution of the differential equation satisfying the boundary conditions can be obtained by the separations of variables. The result is coincident with that derived from the mirror image method. Some singular integrals are interpreted physically from the viewpoint of the theory of the generalized function.