The current drive by electron cyclotron resonance heating (ECRH) is investigated in a Tokamak with arbitrary aspect ratio. The r.f. power is assumed to be weak and the wave-induced current is calculated by using an adjoint equation to the steady-state linearized Fokker-Planck equation with a quasi-linear diffusion term. The trapped electrons and the modification of electron cyclotron resonance condition by the relativistic mass increase are shown to have significant effects on the efficiency of this current drive. The efficiency strongly depends on the values of the parallel velocity u0 of resonant electrons, the inverse aspect ratio in , the poloidal angle theta 0 of the absorption point, and the relativistic parameter S which represents the strength of the relativistic correction to the resonance condition. It is also pointed out that the electron-electron collisions enhance the trapping effects compared with those of the Lorentz gas model.