This paper revisits a classical problem to derive a marginal condition for the onset of spontaneous thermoacoustic oscillations of a gas in a circular tube with one end open and the other closed by a flat wall, subjected to a temperature gradient along the side wall. Formulation is given in the framework of the linear theory and the first-order theory in the ratio of a boundary-layer thickness to the tube radius. An eigenvalue problem is posed on the second-order differential equation with variable coefficients of the axial coordinate for the excess pressure in the main-flow region outside of the boundary layer. A boundary layer on the end wall is taken into account in the form of an appropriate boundary condition. By using the idea of renormalization, the pressure is rescaled and a complex axial coordinate is introduced so that the pressure equation is transformed into a tractable form. It turns out that the equation includes a factor ͑frequency͒ determined by the product of the local sound speed and the logarithmic temperature gradient, though the boundary-layer effect is not completely eliminated. When this factor is constant everywhere, the temperature distribution is parabolic and solutions are obtained in a closed form. The frequency equation is then derived from the boundary conditions at both ends of the tube, from which the marginal condition is obtained for the ratio of temperature at the hot, closed end to the one at the cold, open end against the tube radius relative to the thickness of the boundary layer at the open end. The spatial modes of the oscillations are also analytically obtained and their profiles are displayed. Finally, some discussions on marginal oscillations are given from the viewpoint of energy balance.