Background: Lignocellulosic biomass from plant biomass, especially softwoods, are well-known to present difficulties during attempts at hydrolysis due to their rigid structure. Pretreatment of lignocellulosic biomass with ionic liquids (ILs) is attractive as this requires to a low input of energy. However, IL pretreatment has the disadvantage of the presence of large amounts of water. Recently, it was reported that a small amount of acid has a positive effect on the degradation of biomass in IL with water. In this study the pretreatment of Japanese cedar, the most abundant softwood in Japan, was investigated using a combination of IL, acid and metal ions. Results: First, the novel ionic liquid pretreatment was investigated by changing the pretreatment solvent and the anti-solvent. A mixture of IL, acid and ferric oxide (Fe3+) ion was most effective for pretreatment, and an acetone-water mixture was also most effective on the precipitation of biomass. These optimized pretreatment combinations attained a higher degree of glucose release from the pretreated biomass. The amount of cellulose was concentrated from to a level of 36 to 84\% of the insoluble fraction by the optimized pretreatment. Based on this result, it was assumed that the extraction of the lignin fraction from the biomass into an anti-solvent solution was attained. Finally, this optimized pretreatment was applied to the enzymatic hydrolysis of Japanese cedar at high-solid biomass loading, and 110 g/L of glucose production was attained. In addition, the ethanol fermentation with this hydrolyzed solution by Saccharomyces cerevisiae achieved 50 g/L ethanol production, and this yield reached 90\% of the theoretical yield. Conclusions: We developed an effective pretreatment protocol by changing to a pretreatment solvent containing IL, acid, metal ion and anti-solvent. The optimized pretreatment has an effect on softwood and separately retrieved lignin as a by-product. The saccharified solution at high-solid biomass loading was converted to ethanol in a high yield. This proposed methodology would boost the performance of the bioconversion of low-cost materials to other chemicals, and would not be limited to only ethanol but also would include other target chemicals.