Mechanical removing processes of a mushy structure in an aqueous solution are observed. A rotating cutter moves along a horizontal cold flat surface continuously to remove the mushy structure. The mushy structure stacks on the tip of a cutter and floats up periodically in every 2-4 minutes. The amount of energy to remove the mushy structure from the cold surface is measured experimentally. In a rotating cutter method, the amount of required energy per unit produced ice mass is large when the rotating speed is large. The experimental results are compared with those of a reciprocating cutter method. A rotating cutter method is superior to a reciprocating cutter method in terms of area of a cold surface per unit ice production rate. When the cold surface area is larger than 620cm², the rotating method is superior to reciprocating method in terms of energy required to remove the mushy structure from the cold surface. The final diagram shows the relations among ice production demands, designing conditions and operating conditions of an ice production unit, which leads to the optimal designing and operating conditions.