PURPOSE: To obtain citric acid with a little by-product, by immobilizing a microbial cell of a strain, having the ability to produce citric acid, and obtained by preliminarily cultivated to a gelatinous carrier, and catalytically reacting the immobilized microbial cell with a saccharide solution.

CONSTITUTION: A microbial cell or spore of a strain, belonging to the genus Aspergillus or Candida, and having the ability to produce citric acid, and preliminarily cultivated is entrapped and immobilized in a gelatinous carrier selected from polyacrylamide, carrageenan and alginic acid, and a solution of glucose, sucrose or maltose or an extract from a natural product containing th saccharides as a saccharide solution is brought into contact with the immobilized microorganism. The saccharide solution is brought into contact and reacted with the immobilized microorganism while supplied continuously or intermittently to prepare the aimed citric acid.