We elucidate some exact relations between light-cone and covariant string field theories on the basis of the homological perturbation lemma for A ∞ . The covariant string field splits into the light-cone string field and trivial excitations of BRST quartets: The latter generates the gauge symmetry and covariance. We first show that the reduction of gauge degrees can be performed by applying the lemma, which gives a refined version of the no-ghost theorem of covariant strings. Then, we demonstrate that after the reduction, gauge-fixed theory can be regarded as a kind of effective field theory and it provides an exact gauge-fixing procedure taking into account interactions. As a result, a novel lightcone string field theory is obtained from Witten's open string field theory.1 In this direction, there are some interesting investigations. See [4-6] for example.show that a novel light-cone string field theory appears within the covariant formulation, from which we cannot extract the old light-cone formulation itself directly.Note that gauge-fixed theory can be regarded as a kind of effective theory. For a given gauge theory S[Ψ], one can obtain a gauge-fixed action S red [ψ] by integrating out the gauge degrees ψ g of Ψ = ψ + ψ g as( 1.1) where Vol g denotes its gauge volume. It will give an exact gauge-fixing procedure taking into account interactions. The reduced action S red [ψ] reproduces the same amplitudes as the original action S[Ψ]. As we will show, the homological perturbation lemma provides us exact treatment of this formal procedure. In particular, by applying the lemma for A ∞ algebras, an A ∞ effective field theory is directly obtained from the original A ∞ field theory. In this paper, we construct an action for light-cone string field theory explicitly as the classical part of such an effective action for the Witten's theory.In section 2, we review the relation between the BRST operator and the light-cone kinetic operator. There exist similarity transformations connecting these. In section 3, we explain the homological perturbation lemma for A ∞ . We show that the reduction of gauge degrees can be described by applying the lemma, which provides a refined version of the no-ghost theorem of covariant strings. In section 4, we explain the light-cone reduction of interacting theory. A novel light-cone string field theory is constructed, which has an A ∞ action. Appendix A is devoted to explaining basic facts of the homological perturbation and its application to similarity transformations.In this paper, we write [[A, B]] for the graded commutator of A and B,where the upper index of (−) A denotes A's degree. The graded commutator will be defined for states, operators or mathematical operations appropriately.