PURPOSE: To make it possible to return oil effectively from a flooded type evaporator to a compressor, while the kinds of equipment is reduced and constitution of pipings is simplified.

CONSTITUTION: A partition body 11 extending in a length direction is provided on one inner side of a horizontally long shell 5 to divide the liquid zone in the shell 5 into a main evaporating section 51 and an auxiliary evaporating section 52, and a refrigerant liquid inlet port 7 is provided in the section 51 to introduce refrigerant liquid from the section 51 into the section 52. Heat exchange tubes disposed in the section 52 are connected to cold water inlet pipes 61 to form an oil concentrating section and an oil return pipe 12 is connected to the liquid region of the section 52 to concentrate the oil in a flooded type evaporator so that refrigerant liquid rich with oil can be returned to a compressor.