The selective condensation of 2-methylfuran and furfural to C-15 compounds is performed using supported phosphotungstic acid catalysts in a continuous-flow fixed-bed reactor. The formation of unwanted viscous tet-ramers (C-20) is suppressed when the continuous-flow reaction system is used. Stronger acids, as measured by plc, improved the condensation of furans, but the number of acid sites was rather weakly correlated with the catalytic activity. Suppressed catalytic activity in tiny pores smaller than 2.8-6.3 nm was also observed. Based on the observations of the acidity and pore structures, a strategy with which to design optimum catalysts for the condensation of furan compounds is suggested.