Among renewable and sustainable feedstock to replace fossil-based resources, marine biomass, macroalgae can be preferably considered due to several reasons: First, they contain an abundance of carbohydrate sources that are readily separated from the original feedstock. Second they are naturally well-grown thus multiple cropping is possible. Third, their cultivation techniques have been historically solidly established. Finally, it is known that their CO2 remediation effect is much higher than that of landplants. Agar is a major carbohydrate source in macroalgae and it possesses around 70 wt%. In this study, we tried to directly transform agar into 5-hydroxymethyl-2-furfural (HMF) without saccharification step. In order to find best catalytic reaction conditions such as metal catalysts, solvents, acidity, time, temperature, high throughput screening was extensively performed. A range of metal catalysts were screened and 1-ethyl-3-methylimidazolium(EMIM) salt, so called ionic liquid and aprotic polar DMSO were tested as solvents. When RuCl3 was used as a metal catalyst, 6.4 g/L of HMF could be obtained from 10 wt% agar solution.