PROBLEM TO BE SOLVED: To provide a method of producing plain-type lactic acid fermented soybean milk such that a rate of change in lactic acid acidity during product storage becomes 20% or less.SOLUTION: This invention provides a method of producing plain-type lactic acid fermented soybean milk by the fermentation of a preparation liquid comprising soybean milk, vitamin B12 requiring lactic acid bacteria and vitamin B12, where vitamin B12 content in the preparation liquid is controlled such that a rise rate in lactic acid acidity during a product storage period becomes 20% or less.
