Recently, it has been realized that the soil pH of some farmlands in Japan has decreased. Therefore, in this study, we clarified this phenomenon using two soil conditioners, calcium carbonate (CC) and sulfur trioxide (ST). The purpose was to investigate the effects of soil pH changes induced by these conditioners on rice straw decomposition and methane emission from paddy fields in Japan’s cold regions. We applied CC to 10 t ha⁻¹ and ST to 12 t ha⁻¹, and measured methane flux by the closed chamber method in 2015 and 2016. As a result, soil pH reached 7.0 and 5.2 in CC and ST plots, respectively, but remained at 5.5 in the control plot. From October 2014 to April 2015, rice straw decomposition rates in CC, ST, and control plots were 34.5, 27.8, and 31.8％, respectively. The CC plot had the highest methane emission rate. On the other hand, the methane emission rate was the lowest in the ST plot due to sulfate reduction. In 2015, methane emission rates were 37.8, 11.7, and 31.4 g CH₄ m⁻² in CC, ST, and control plots, respectively. Based on this study, it is important to determine the components of soil conditioners and adjust soil pH so that the decomposition of rice straw and methane emission from paddy fields are balanced.