PURPOSE:To secure such an air-cooled fin tube cooler that is compact in size and inexpensive in cost, by making cooling air to be generated by a cooling fan of a motor frame, also flow past in a fin tuber guiding lubricating oil. CONSTITUTION:At a rear end of a motor part 14, a cooling fan 18 is pivotally connected to a turning shaft 16 of a motor, and the outer circumference is constituted so as to surround it with a cover 20 and to form a specified ventila tion flue. And, a fin tube 26 is surroundedly set up in the outer circumference of a coupling part between the motor part 14 and a speed-up mechanism part 12, and it is constituted so as to cause cooling air flowing down along the outer circumferential surface of a motor frame to come into contact with this fin tube 26. In this case, lubricating oil to be fed to the inside of the speed-up mechanism part 12 is made to flow back to the inside of the fin tube 26.
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