Magnon propagation length in a ferrimagnetic insulator yttrium iron garnet (YIG) has been investigated by measuring and analyzing the YIG-thicknesst(YIG)dependence of the spin Peltier effect (SPE) in a Pt/YIG junction system. By means of the lock-in thermography technique, we measured the spatial distribution of the SPE-induced temperature modulation in the Pt/YIG system with thet(YIG)gradation, allowing us to obtain the accuratet(YIG)dependence of SPE with hight(YIG)resolution. Based on thet(YIG)dependence of SPE, we verified the applicability of several phenomenological models to estimate the magnon diffusion length in YIG.