We theoretically propose a gigantic orbital Edelstein effect in topological insulators and interpret the results in terms of topological surface currents. We numerically calculate the orbital Edelstein effect for a model of a three-dimensional Chern insulator as an example. Furthermore, we calculate the orbital Edelstein effect as a surface quantity using a surface Hamiltonian of a topological insulator, and numerically show that it well describes the results by direct numerical calculation. We find that the orbital Edelstein effect depends on the local crystal structure of the surface, which shows that the orbital Edelstein effect cannot be defined as a bulk quantity. We propose that Chern insulators and Z_2 topological insulators can be a platform with a large orbital Edelstein effect because current flows only along the surface. We also propose candidate topological insulators for this effect. As a result, the orbital magnetization as a response to the current is much larger in topological insulators than that in metals by many orders of magnitude.
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