Soybean is known to contain various phytochemicals that are related to anti-oxidant, anti-inflammatory and anti-obesity effects in mice and humans. The anti-obesity effect of by-product extracts from soybean on the differentiation of 3T3-L1 pre-adipocytes to adipocytes was investigated by suppressing adipocyte differentiation and lipid accumulation with Oil Red-O assay and quantitative PCR. In inducing differentiation of 3T3-L1 pre-adipocytes in the presence of an adipogenic cocktail, isobutylmethylanthine (IBMX), dexamathasone, and insulin, treatment with filtrated soybean soaked water, soybean milk, and soycurd residue from soybean curd processing significantly decreased mRNA expression of obesity-related gene such as PPAR${gamma}$, Fabp4, and Scd1, adipsin, apolipoprotein (APOE) and adiponectin (ADIPOQ) without any significant cytotoxicity. We also determined the well-known isoflavones in soybean, such as daidzein and genistein, in the by-product extracts. Taken together, we suggest that soybean by-product extract showed anti-obesity effect by suppressing adipocyte related gene expression, and that by-products collected during soybean curd processing may be a good candidate as an ingredient in health care products.