"Kyouzaikenkyuu" (translated as "instructional materials research") is said to be a crucial part of successful Japanese lesson study. "Kyouzaikenkyuu" is described as the planning portion of the research lesson during lesson study. "Kyouzaikenkyuu" is also said to be done on a daily basis by Japanese teachers; however, there is very little written about this process in the literature. This study takes an in-depth look into what Japanese junior high mathematics teachers do during their daily "kyouzaikenkyuu," and why they do it. Through interviews, observations, and participation in "kyouzaikenkyuu," we explain the process and key ideas that many Japanese mathematics teachers go through to prepare their lessons. We document two different approaches to "kyouzaikenkyuu." We also found that for some teachers there is a large difference between what they do during "kyouzaikenkyuu" for a research lesson compared to the "kyouzaikenkyuu" of a daily lesson. If teachers outside of Japan wish to engage in "kyouzaikenkyuu" and lesson study, these approaches are a good place to start.