The present invention relates to a process for the fermenta­tive preparation of lactic acid, by allowing a sterilized growth medium to be continuously fermented in a reaction vessel by a culture of bacilli of the genus Bacillus , which form lactic acid, keeping the pH of the reaction mixture at 3 to 9 with an aqueous solution of XOH (where X is NH₄ or a metal whose hydroxide and whose salt of lactic acid are watersoluble), subjecting the fer­mented reaction mixture to an ultrafiltration, recycling the reten­tate to the reaction vessel, and concentrating the permeate and subsequently subjecting it to an electrodialysis wherein bipolar membranes are used, in which process the lactate is decomposed into the lactic acid and XOH.