For a field F of characteristic 0 and an additive subgroup Γ of F, there corresponds a Lie algebra W(Γ) of generalized Weyl type. Given a total order of Γ and a weight Λ, a generalized Verma W(Γ)-module M (Λ, ≺) is defined. In this paper, the irreducibility of M (Λ, ≺) is completely determined. It is also proved that an irreducible highest weight module over the W-infinity algebra W1+∞ is quasifinite if and only if it is a proper quotient of a Verma module.