PROBLEM TO BE SOLVED: To provide a novel method of accurately and securely heat-sterilizing an injection liquid in manufacturing the injection liquid containing hyaluronic acids suitable for drugs from hyaluronic acids whose molecular weight is from 1.5 million to 4 million.SOLUTION: An indirect heating device is used in the method. The indirect heating device includes an inner tube through which a heated liquid flows, and an outer tube disposed to separately enclose the outer periphery of the inner tube. A heating medium passage is formed in the space between the inner and outer tubes, and a plurality of fixed agitator mechanisms, each of which includes a shaft with a length in the axial direction and a plurality of blades projecting in the radial direction from the shaft and having angles in the axial direction, are separately disposed in the axial direction in the indirect heating device.
