Studying mineral properties under extreme environments is essential for understanding the processes occurring deep inside the Earth. In order to simulate the high-temperature and high-pressure conditions expected inside the Earth, combined usage of a laser heating system and a diamond anvil cell has been actively developed and established as a standard means in mineral physics research. Reproducing such simultaneous high-temperature and high-pressure environments involves the use of extremely small amounts of sample in a few tens of microns in dimension, which in turn requires high precision and stability of the laser heating system. For this purpose, a double-sided laser heating system has been constructed consisting of a fiber laser, an optical lens system that enables micron-level focusing control, and a spectrometer for temperature measurement. In this paper, we introduce the configuration of a newly constructed double-sided laser heating system and verify its application through a phase transition experiment of feldspar mineral that occurs under the mantle transition zone conditions. The quality of the diffraction data measured for the sample recovered after laser heating under pressure allows the evaluation of the homogeneous phase transition in the heated portion and subsequent analysis using the Rietveld analysis. Such a laboratory-based laser heating system is expected to increase the efficiency of extreme environment experiments at synchrotron facilities.