provided on a radial arm 27 projecting toward the inside of an oil inlet port 26 bored on the case 22 whereas at the other is provided a drive pulley 29 which is driven by a driving source not shown to rotate the impeller shaft 18 on the other hand there are arranged the water jackets 11 between said impellers 4 coaxially with respective impellers 4 but not contacting therewith the water jacket 11a on the driving side is fixed inside the case 22 whereas the other water jackets 11 11 positioned alternately with the impellers 4 in a sequence are fixed to the said water jacket 11a by placing spacers 33 between the lug 16 the water inlet port 31 and the outlet port 32 to keep them apart by predetermined distances and by tightening the bolt 30 placed through the lug 16 by a nut not shown one end of the inlet port 31 and that of the outlet port 32 for the cooling water are respectively tightened by a cap nut 34 which also acts as a blank cap there is bored a hole 35 for passing the water to the cooling water pipes 31 and 32 to correspond to the distance between the water jackets 11 and the spacers 33.