The present invention provides, in the preparation of a hot press molded board from a lignocellulose or an inorganic material as a main raw material, an isocyanate-based binder composition with which a board having eternally good releasability from a hot press plate, excellent physical properties and a low hygroscopic linear expansion coefficient can be obtained with improved productivity. The binder composition comprises (A) a compound having at least two isocyanate groups, (B) a low molecular weight polyethylene and (C) water as indispensable components.