To provide a method that reduces clogging of a filter device (filter) during a filtering step prior to addition of milk to a previously formulated lactase solution, and to provide a lactase solution which is not prone to having an occurrence of clogging of the filter device. A lactase solution having a permeation rate of 5 kg/min×m2 or more at the time of permeation of 366 kg/m2 through a membrane filter with a pore diameter of 0.22 μm at a concentration for having an activity of 1,000 NLU/g or 2,000 ALU/g, after being subjected to a stirring treatment at 100 rpm, 10° C. for 16 hours with a concentration for having an activity of 5,000 NLU/g or 10,000 ALU/g.
