To reduce rotational loss of a rotary body while surely lubricating slide parts in a fluid machine such as a hydraulic pump, a hydraulic motor, a hydrostatic transmission or the like.

In a hydrostatic transmission in which a hydraulic pump mechanism 2 is incorporated in the upper part of the inside of a closed container 11 while a hydraulic motor mechanism 3 is incorporated in the lower part thereof, a position where a drain port is formed is at a height which is substantially the same as that of an input shaft 26 of the hydraulic pump mechanism 2. The level of hydraulic oil in the closed container 11 is set to a vertically intermediate position of the hydraulic pump mechanism 2, thereby it is possible to reduce rotational loss of plungers 22 in the hydraulic pump mechanism 2 when they are rotated.