The bed form of steep channels are often characterized by a series of steps. In this paper, in order to investigate the mechanism of the inception of steps, a linear stability analysis is performed with the use of the two-dimensional Reynolds equations and the Exner equation for bed erosion. The analysis reveals that the flat bed becomes unstable resulting in the growth of sinusoidal perturbations when the Froude number is larger than 0.6-0.9 and the wavenumber is smaller than 0.7-1.0. The characteristic wavenumber of inceptive steps are in the order of 0.1-1, corresponding to a wavelength of 6-60 times the flow depth. The comparison between the predicted wavenumbers in this analysis and the measurements in Sawai's experiments shows a reasonably good agreement. The analysis can also explain that the steps migrate upstream, which agrees with the observational results.