Electron cyclotron wave current startup has been investigated in a wide range of toroidal magnetic fields on the CT-6B tokamak. Two kinds of drive mechanisms of current startup have been identified experimentally, according to the dependence of the wave started current on the vertical field. Their contributions to the current have been estimated quantitatively for different resonance scenarios. The wave started current may occur at a downshifted resonance frequency even if the resonance layer is located outside the vacuum vessel.