  We report magnetization measurements under high hydrostatic pressure in the
newly discovered pyrochlore superconductor RbOs_2O_6 (T_c\simeq6.3K at p=0). A
pronounced and {\it positive} pressure effect (PE) on T_c with dT_c/dp
=0.090(1)K/kbar was observed, whereas no PE on the magnetic penetration depth
\lambda was detected. The relative pressure shift of T_c [ dlnT_c/dp \simeq
1.5%/kbar] is comparable with the highest values obtained for highly underdoped
high-temperature cuprate superconductors. Our results suggest that RbOs_2O_6 is
an adiabatic BCS-type superconductor.
