To develop reaction systems of chemical conversion of biomass, fructose was heated in an aqueous medium under pressurized CO2, which caused in situ generation of carbonic acid. It gave 5-hydroxymethyl-2-furfural (HMF) as a dehydration product in good yields. The maximum yield of HMF was 92% in the reaction at 90 °C under 7.0 MPa of CO2 for 168 h. The reaction proceeded in the absence of any other reagents such as non-volatile acids, organic solvents, inorganic, and organic salts. It means green, environmental-friendly, and efficient process was achieved for production of HMF.