The reradiation characteristics of a grating coupler for a grounded dielectric slab waveguide are investigated for the transverse magnetic mode, taking into account the finite conductivity of the ground plane. A pair of canonical equations for the coupler are derived. Analyses based on the solution of the canonical equations show the effects of the finite conductivity on the minimum reradiation coefficient, the optimum modulation index of the grating, the optimum incident angle, and the sensitivity of the reradiation to the changes in the incident angle about the optimum value. The coupler with the grating on the conducting surface is compared with the coupler having the grating on the film-cover interface for both the transverse-magnetic and transverseelectric modes.