The anthelmintic substance of the roasted fruits of Prunas mume against Clonorchis sinensis was isolated and its structure was identified by chemical and physical methods. The results obtained from the experiments are as follows: 1) The methanol extract of the roasted fruits of P. mume was fractionated into four parts: petroleum ether, ethyl ether, ethyl acetate and water soluble part. Among these, etherial fraction was found to have strong wormicidal effect on liberated metacercaria of Clonorchis sinensis. 2) From the etherial fraction, the wormicidal substance was isolated by silica gel, polyamide and sephadex column chromatography and identified to be 5-hydroxymethylfurfural (5-HMF) by chemical and spectral data. 3) 5-HMF was synthesized and administered to the rabbits infected with C. sinensis. On the 2nd day after administration, the EPG (eggs per gram in feces) reached to the maximal value. Among the adult worms isolated from the bile duct of the treated animal, 84% of worms were damaged morphologically. 4) The content of 5-HMF in the fruits of P. mume which were roasted in an oven at $90-110^{circ}C$ for 52 hours and that in the fresh fruits was evaluated by HPLC. The content of 5-HMF was 0.8% in the roasted fruits and 0.02% in the fresh ones.