Abstract: [Problem to be Solved] To provide a lactase solution with good heat stability as desired by identifying a factor to influence heat stability in a lactase product and using the factor as an index. [Solution] Proteinase B having the following enzymatic properties, and a lactase solution containing less than 11.5 ng of the proteinase B. 1) having an optimum temperature of about 40° C., 2) having an optimum pH of about 8.0, 3) being stable at pH 5.0 to 8.0, 4) having high substrate specificity to FITC-casein and lactase, 5) having a neutral lactase-fragmenting action, and 6) having a molecular weight of about 29,700 to 30,000 (by SDS-PAGE).