The Clapeyron slope of the perovskite to post-perovskite phase transition has not been well determined experimentally. Crystal structure of CaIrO3 is equivalent to the MgSiO3 post-perovskite phase (space group: Cmcm) at ambient condition. We have found that perovskite phase (Pbnm) of CaIrO3, which is isostructural with MgSiO3 perovskite, is stable at 1--3 GPa and 1400--1550¿C. The perovskite to post-perovskite phase transition in CaIrO3 was determined by using piston-cylinder apparatus at 0.9--3.0 GPa and 1350--1550¿C. Results show that phase transition pressure is strongly temperature-sensitive with a Clapeyron slope of +17(¿3) MPa/¿C. These results, obtained for a suitable analogue material, suggest that post-perovskite phase transition in MgSiO3 also has a large positive value of the Clapeyron slope. Such a pronounced exothermic reaction in the Earth's lowermost mantle will promote the upwelling of high-temperature plumes from the core-mantle boundary region.