Suppression of knock induced by the auto-ignition of the end gas in spark ignition engines is very important for improvement of thermal efficiency. Occurrence of knock is usually suppressed by inhibiting the auto-ignition of the fuel-air mixture. However, knock does not occur if the pressure oscillation induced by the auto-ignition is sufficiently reduced. It is thought that the pressure oscillation occurs when the rising of the pressure by the auto-ignition of the local mixture is not adequately decreased by the expansion of the gas to the surroundings. It indicates that a reaction rate of the auto-ignition affects the pressure oscillation, namely knock intensity. In this paper, the knock intensity of the diluted fuel-air mixture was measured with varying the dilution ratio and the dilution gas in order to change a reaction rate of the auto-ignition of the mixture by using a rapid compression machine and a singly cylinder spark ignition engine. The condition of reducing knock intensity was investigated. In the case of low dilution, the heavy knock intensity was observed when the auto-ignition of the end gas occurred. With increase in the dilution ratio, the knock intensity decreased. These were observed in either case that dilution gas was inert gas or air. The knock intensity decreased with decrease in the maximum temperature and the maximum pressure, although the knock intensity was influenced by the composition of the mixture, load, ignition timing, auto-ignition timing of the end gas and heat release quantity of the auto-ignited mixture. The temperature and the pressure at the timing that auto-ignition occurs, which affects the high temperature oxidation reaction rate, had a good correlation with the maximum temperature and the maximum pressure. It is thought that this is why the knock intensity was shown as a function of the maximum temperature and the pressure.