Rb-substitution effect on the metal-insulator transition of hollandite vanadate, K₂V₈O₁₆ has been investigated. The metal-insulator transition is accompanied by a charge ordering and a structural change from a tetragonal to a monoclinic structure with an increase of volume in the low-temperature monoclinic insulator phase. Rb-substitution for K atoms continuously raises the transition temperature up to 240 K for Rb₂V₈O₁₆ from 160 K for K₂V₈O₁₆ and does not make the transition broad in contrast to Ti-substitution. This significant rise of transition temperature can be understood as a negative chemical pressure effect caused by Rb-substitution.