A crystalline constituent, futoenone; C_<20>H_<20>O_5, m.p. 197°, was isolated from Piper futokadzura and proved to have the structure 1. On the basis of uv spectra of futoenone (1), dihydrofutoenone (3) and the hydroxy compounds (2,4), the presence of a piperonyl and α,β-unsaturated carbonyl moiety were suggested as the chromophores. The structure of diacetate (5), given by the treatment of futoenone with acid in acetic anhydride, was designated as 5 by means of n.m.d.r. and chemical evidences. The result of giving back to futoenone from the diacetate (5) via tosyl-phenol (9) or anisyl-alcohol (10), providing an interesting example of Ar_1-6 participation, strongly supported the spirodienone structure in futoenone. Lemieux oxidation of futoenone and hydrogen peroxide oxidation of demethylfutoenone (13) gave γ-lactone (15) and (16) respectively. This result and the formation of the highly conjugated benzofuran derivative (12) by oxidation of the diacetate (5) with DDQ indicated the presence of a fused tetrahydrofuran ring. N.m.d.r. studies of futoenone (Table 3) deduced a partial structure D for the aromatic ring, E for the saturated carbons and F for the dienone moiety. The three partial structure together account for nineteen carbons, all of the hydrogens and oxygens in futoenone. Since the carbon atom linked with C-5 and C-1 cannot bear any proton and C-11 carbon cannot link any carbon having hydrogen, C-11 carbon must be bonded with an oxygen and C-1, C-5, C-7 and C-11 carbons must all be linked to a spiro carbon to deduce the full structure of futoenone. The stereochemistry was also proposed by the data of n.m.r.