The Belousov-Zhabotinsky (BZ) reaction in a gel system was studied as a function of initial concentration of substrates (malonic acid, sodium bromate, and nitric acid), through which the understanding of the reaction mechanism has been attempted. We prepared gels composed of cross-linked N-isopropylacrylamide networks to which ruthenium catalyst was covalently bonded. To compare the period and waveform of the BZ oscillations in the solution with those in the gel, an image processing method was developed and applied to the gel system. We monitored the oscillating change in the mole fraction of oxidized ruthenium catalyst. The period dependence on the initial substrate concentrations showed that there is a marked difference between the solution and gel systems. In particular, malonic acid and NaBrO3 affect the period over their low concentration ranges. A marked effect of malonic acid concentration appeared in the waveform in the gel but not in the solution. By analyzing these results with the Field-Koros-Noyes (FKN) mechanism, diffusion limitation of substrate and product was found to play an important role in the oxidizing and reducing rates of the ruthenium catalyst within the gel system.