Biomass is expected to be an alternative resource to fossil resources. In this study, the development of a biomass conversion method into the valuable chemical, reducing sugar, was examined. For the conversion, thermochemical technology was focused on for its advantage of short reaction time, and ionic liquid was focused on as a reagent to overcome the unpreferable disadvantage of thermochemical technology, which is the low selectivity. Cedar and crystalline cellulose were pretreated with ionic liquid for reforming into desirable precursors of reducing sugar. Especially when they were pretreated by 1-ethyl-3-methylimidazolium methylphosphonate at 150 degrees C for 1 h, the pretreatment worked effectively by decreasing the crystallinity of samples. Pretreated cedar and crystalline cellulose were converted into reducing sugar under hydrothermal conditions, respectively, by 39 C-% and 90 C-%. Recovery of ionic liquid was also examined. When cedar was used as a material, lignin was dissolved into ionic liquid through pretreatment, which was undesirable because of difficult separation thereof. When crystalline cellulose was used as a material, 98.3% of the ionic liquid was recovered after the conversion with the highest yield of reducing sugar (90 C-%).