A liquid coolant supply pipe and an over flow pipe are provided on the outer side of a capsule forming pipe through which a liquid coolant flows and into which liquid droplets are dropped from a nozzle. The liquid coolant is supplied to the liquid coolant supply pipe by a low pulsatory motion type pump, and the flow is regulated by a flow straightening block in the liquid coolant supply pipe. The cooling liquid passes through a liquid coolant introducing section having a curved surface, and then flows into the capsule forming pipe from an upper portion opening in a cap mounted to the upper part of the capsule forming pipe. The liquid coolant flows into the capsule forming pipe, and the excess flows into the overflow pipe.