PURPOSE: To handle fluid which is apt to have a thermal effect properly by forming a heater and a refrigerant flow passage at a pump body, and using or switching the opening condition of refrigerant introduced in the heater and the refrigerant flow passage for keeping the temperature of the inside of the pump body roughly constant.

CONSTITUTION: A heater 7 and a refrigerant flow passage 8 are formed inside a casing 1. The heater 7 is a rod-shaped cartridge heater, where a cartridge heater 7 is inserted into a hole 1a drilled in the casing 1 to be embedded. At the six faces of the casing 1, a hole 1b is punched toward an opposed face from one face, or drilled on the way. The hole 1b is crossed in a roughly cubic shape inside the casing 1 to form the refrigerant flow passage 8. By carrying out switching between an energizing condition to the heater 7 and the introducing condition of refrigerant into the refrigerant flow passage 8 as necessary in this gear pump, above all, it is possible to hold the temperature in the casing 1 at the peripheral parts of gears 4a, 4b through which melt plastic passes, to a roughly optimum value.