closed fluid circuit of the hst even when charge pump 45 is not driven check valve 47 communicates at the inlet side thereof with intake port 45 b through an oil passage 46 h and at the outlet side with recess 5 g through an oil passage 46 i when charge pump 45 is driven to flow operating oil into recess 5 g though oil passages 46 c and 46 e check valve 47 closes between oil passage 46 h and oil passage 46 i when charge pump 45 is not driven causing negative pressure on the low pressure side of the closed circuit check valve 47 is open to enable oil filtered by filter 56 to be guided from intake port 45 b and oil passages 46 h and 46 i into recess 5 g check valve 54 or 55 at the negative pressure side of the closed fluid circuit is open through oil passage 5 d whereby oil is supplied to the closed fluid circuit thus oil can be maintained in the closed fluid circuit at all times in order to place operating oil into the closed fluid circuit after the axle driving apparatus is assembled oiling pipes 52 and 53 are disposed at the lower surface of center section 5 as shown in fig1 15 17 and 20 at the lower surface of center section 5 a fourth vertical passage 5 e is bored to communicate with the upper deep portion of arcuate port 41 a a fifth vertical oil passage 5 f is bored to communicate with second oil passage 5 b oiling pipes 52 and 53 are mounted into oil passages 5.