. IntroductionLet X be an algebraic surface (over C) embedded in the projective space P N (C) and S be its singularity set. If S is empty, then the harmonic spaces M l (X) = {smooth /-form a on X\da = da=0} (or, equivalently, its de Rham cohomology groups) have the pure Hodge structure;rally changed into the Dolbeault-type harmonic space M P^q (X) in this case, but in the case we are going to discuss in this paper, that is, in the case where S is not empty, such a change has a subtle problem ( § 3) and it seems to be one of the key points not to try to do so.) Also there exists the hard Lefschetz structure compatible with (1.1); The author believes that the case i=2 omitted in Theorem 1(2) and in the above remark must hold.Conjecture A. Theorem 1(2) and the hard Lefschetz decomposition of the L 2 -cohomology groups hold also in the case i=p+q=2.Obviously this conjecture is equivalent to a part of it, i.e., M Moreover it suffices to prove a little bit stronger assertion; Ker ^= which is equivalent to Ker rf 2 =Ker rf^2. Thus Conjecture A can be deduced from the conjecture "d^i=d i for i=l 9 2" announced in [10].Acknowledgement. The author would like to thank Professors J. Noguchi, M. Oka and Y. Miyaoka for offering him valuable suggestions during the preparation of this paper. Also he would like to thank the referee for valuable comments.