An isocyanate-based one-part moisture curable composition having excellent storage stability and high curing rate, and use of this composition enables production of a cured product of high flexibility without using any plasticizer. The composition comprises (A) an isocyanate compound having a structure wherein all of the isocyanate (NCO) groups in the molecule have secondary or tertiary carbon bonded thereto, and (B) a ketimine having ketimine (C═N) bond derived from a ketone or an aldehyde and an amine having a structure wherein &agr;-position of at least one of the carbon or the nitrogen of the ketimine bond has a branched carbon atom or a ring member carbon atom bonded thereto. A urethane prepolymer having such structure is preferable for use as the isocyanate compound (A).