We study a depinning transition of a discrete interface growth model for quenched impurities with an external driving force F in d = 2 + 1 dimensions, and determine various critical exponents related to the depinning transition. At the critical force F_c, the growth velocity v of the average interface height follows a power-law behavior v(t) ∼ t^{− δ} with δ≈0.460 and the interface width W shows a scaling behavior W²(t, L) ∼ L^2αf(t/L^z) with α≈1.02 and z≈1.77 where L is the system size. The steady-state moving velocity v_s of the average interface height shows v_s ∼ (F − F_c)^θ with θ≈0.575 for F > F_c. The lateral correlation length follows a power law ξ_r ∼ |F − F_c|^{− ν_r} with ν_r≈0.71(3) and the correlation time τ ∼ |F − F_c|^{− ν_t} with ν_t≈1.25(5). These exponents are very similar to those of directed percolation universality class.