The phase transition between CaSiO3 perovskite and Ca2SiO4 larnite + CaSi2O5 titanite has been investigated using a Kawai-type multianvil apparatus (KMA) combined with synchrotron radiation. This phase boundary is located by our experiments as P (GPa) = 9.8(4) + 0.0017(3) ¿ T (¿C), which is 2--4 GPa lower than the most recent results using laser heated diamond anvil cell and those based on thermodynamic calculations, but is consistent with earlier studies based on quench experiments using KMA. The present results suggest some diamond inclusions recently found in Kankan distinct, Guinea, may originate from depth of 240--360 km in the upper mantle.