Room-temperature photoreflectance (PR) measurements were successfully used to investigate optical transition energies in InP/InGaAs(1-5 ML)/InP single quantum wells (SQWs) grown by low-pressure organometallic vapor phase epitaxy (OMVPE). PR features due to subband transitions were clearly observed even in the SQWs of extremely thin well thicknesses. PR spectra showed e(1)-hh(1) and e(1)-lh(1) transitions in the InGaAs wells together with the band-to-band transition in the InP layers. Clear PR spectra indicate excellent optical quality of these OMVPE-grown structures. The transition energies were determined by fitting the PR spectra to the theoretical line-shape expression. The resultant e(1)-hh(1) transition energies were equal to energy positions of emission peaks observed in photoluminescence measurements at room temperature. The e(1)-hh(1) and e(1)-lh(1) transition energies decreased with increasing well thicknesses. The behavior agreed qualitatively with the theoretical prediction, although there is a significant discrepancy in transition energies.