A driving force transmission device comprising: a driving source; a clutch unit including a clutch coupled to the driving source and being capable of connecting or disconnecting transmission of driving force from the driving source, and a hydraulic cylinder configured to hydraulically control the engaging and disengaging of the clutch; a transmission unit coupled to the clutch unit, and including a transmission housing, a transmission mechanism built in the transmission housing, and a hydraulic source configured to generate a hydraulic pressure to required portions, a cylinder housing provided in the clutch unit and having a first hydraulic passage to the hydraulic cylinder; and a hydraulic part provided in the transmission unit and having a second hydraulic passage configured to be brought into communication with the first hydraulic passage in the cylinder housing by coupling the clutch unit thereto, whereby the cylinder housing is a stationary member including a cylinder oil chamber that is an oil chamber of the hydraulic cylinder, the cylinder oil chamber and the first hydraulic passage communicate with each other in the cylinder housing, the cylinder housing and the hydraulic part are brought into close contact with each other to bring the first hydraulic passage and the second hydraulic passage into communication with each other, and the clutch unit includes a clutch cover coupled to a transmission input shaft of the transmission unit, a piston slidable in a cylinder hole formed in the cylinder housing, and a piston arm disposed between the piston and the clutch, and provided slidably in a through-hole formed in the clutch cover in such a manner as to generate a pressing force toward the clutch by using the pressure from the piston, a partition elastic member (88, 89) disposed between the piston arm and the clutch and configured to block leak oil from a sliding part of the piston arm from flowing into the clutch, characterized in that a seal member disposed downstream of the partition elastic member (88, 89) in the flow direction of the leak oil, and configured to tightly seal between opposed faces of the cylinder housing and the clutch cover, a leak oil recovery passage (33, 34) using a gap formed between the cylinder housing and the clutch cover as a part of the leak oil recovery passage (33, 34), and configured to carry at least the leak oil from a sliding part of the piston into the transmission unit through the connection between the cylinder housing and the hydraulic part while maintaining a tightly sealed state by the partition elastic member (88, 89) and the seal member. a driving source; a clutch unit including a clutch coupled to the driving source and being capable of connecting or disconnecting transmission of driving force from the driving source, and a hydraulic cylinder configured to hydraulically control the engaging and disengaging of the clutch; a transmission unit coupled to the clutch unit, and including a transmission housing, a transmission mechanism built in the transmission housing, and a hydraulic source configured to generate a hydraulic pressure to required portions, a cylinder housing provided in the clutch unit and having a first hydraulic passage to the hydraulic cylinder; and a hydraulic part provided in the transmission unit and having a second hydraulic passage configured to be brought into communication with the first hydraulic passage in the cylinder housing by coupling the clutch unit thereto, whereby the cylinder housing is a stationary member including a cylinder oil chamber that is an oil chamber of the hydraulic cylinder, the cylinder oil chamber and the first hydraulic passage communicate with each other in the cylinder housing, the cylinder housing and the hydraulic part are brought into close contact with each other to bring the first hydraulic passage and the second hydraulic passage into communication with each other, and the clutch unit includes a clutch cover coupled to a transmission input shaft of the transmission unit, a piston slidable in a cylinder hole formed in the cylinder housing, and a piston arm disposed between the piston and the clutch, and provided slidably in a through-hole formed in the clutch cover in such a manner as to generate a pressing force toward the clutch by using the pressure from the piston, a partition elastic member (88, 89) disposed between the piston arm and the clutch and configured to block leak oil from a sliding part of the piston arm from flowing into the clutch, characterized in that a seal member disposed downstream of the partition elastic member (88, 89) in the flow direction of the leak oil, and configured to tightly seal between opposed faces of the cylinder housing and the clutch cover, a leak oil recovery passage (33, 34) using a gap formed between the cylinder housing and the clutch cover as a part of the leak oil recovery passage (33, 34), and configured to carry at least the leak oil from a sliding part of the piston into the transmission unit through the connection between the cylinder housing and the hydraulic part while maintaining a tightly sealed state by the partition elastic member (88, 89) and the seal member.