The present invention relates to a lactic acid fermented product of cassia seeds. The fermented product of the present invention uses cassia seeds, which have long been used and have proven their safety without causing side effects. Due to this advantage, the lactic acid fermented product of the present invention is effective in increasing the number, water content, and/or weight of feces. The effects of the lactic acid fermented product according to the present invention are comparable to those of sennoside, which is a therapeutic agent for stimulant constipation and whose use in health functional foods is forbidden because of its side effects, and Dulcolax-S, which is currently commercially available and includes bisacodyl and sodium docusate as active ingredients. Therefore, the lactic acid fermented product of the present invention can be utilized in pharmaceutical compositions for treating or preventing constipation or health functional foods for ameliorating or preventing constipation. In addition, the present invention relates to Lactobacillus kefiri strain MJ90. The strain is advantageous for the preparation of the lactic acid fermented product with enhanced activity in the treatment, amelioration or prevention of constipation.