A hydraulic actuator oil passage structure that is able to supply hydraulic oil from an oil source to hydraulic actuators (20, 50, 60, 70, 80) provided in a work vehicle, the structure comprising: a housing that stores traveling-system transmission devices which can be provided in the work vehicle and has a valve block mount surface including first and second regions (161, 162) on an outer surface; a main function valve block 200 -200) accommodating one or more valves suitable for acting on one or more main function hydraulic actuators (20, 50, 60) that are traveling-system hydraulic actuators for changing a drive state of drive wheels in the work vehicle out of various hydraulic actuators (20, 50, 60, 70, 80) mountable in the work vehicle, the main function valve block 200 -200) including a reception port that is suitable to receive hydraulic oil from the oil source, one or more main function relay ports (220P, 260P, 265P, 270P, 275P) suitable to be connected to the main function hydraulic actuators (20, 50, 60), and a removal port from which at least a part of hydraulic oil that has flowed into the reception port of the main function valve block 200 -200) can be taken out; an additional function valve block (300) accommodating one or more valves suitable for acting on one or more additional function hydraulic actuators (70, 80) that are traveling-system hydraulic actuators not classified into the main function hydraulic actuators (20, 50, 60) out of the various hydraulic actuators (20, 50, 60, 70, 80), the additional function valve block (300) including an additional function reception port that is able to receive hydraulic oil and one or more additional function relay ports (330P, 335P, 340P, 345P) suitable to be connected to the additional function hydraulic actuators (70, 80); the main function valve block 200 -200) being mountable on the first region so as to realize a state in which hydraulic oil supplied into the reception port from the oil source is supplied through the main function relay port (220P, 260P, 265P, 270P, 275P) to the main function hydraulic actuator (20, 50, 60) that is operatively controlled by the valve accommodated in the main function valve block (200 -200); and the additional function valve block (300) being mountable on the second region so as to realize a state in which hydraulic oil is supplied through the removal port, a communication oil passage formed in the housing, the additional function reception port and the additional function relay port (330P, 335P, 340P, 345P) to the additional function hydraulic actuator (70, 80) that is operatively controlled by the valve accommodated in the additional function valve block (300). a housing that stores traveling-system transmission devices which can be provided in the work vehicle and has a valve block mount surface including first and second regions (161, 162) on an outer surface; a main function valve block 200 -200) accommodating one or more valves suitable for acting on one or more main function hydraulic actuators (20, 50, 60) that are traveling-system hydraulic actuators for changing a drive state of drive wheels in the work vehicle out of various hydraulic actuators (20, 50, 60, 70, 80) mountable in the work vehicle, the main function valve block 200 -200) including a reception port that is suitable to receive hydraulic oil from the oil source, one or more main function relay ports (220P, 260P, 265P, 270P, 275P) suitable to be connected to the main function hydraulic actuators (20, 50, 60), and a removal port from which at least a part of hydraulic oil that has flowed into the reception port of the main function valve block 200 -200) can be taken out; an additional function valve block (300) accommodating one or more valves suitable for acting on one or more additional function hydraulic actuators (70, 80) that are traveling-system hydraulic actuators not classified into the main function hydraulic actuators (20, 50, 60) out of the various hydraulic actuators (20, 50, 60, 70, 80), the additional function valve block (300) including an additional function reception port that is able to receive hydraulic oil and one or more additional function relay ports (330P, 335P, 340P, 345P) suitable to be connected to the additional function hydraulic actuators (70, 80); the main function valve block 200 -200) being mountable on the first region so as to realize a state in which hydraulic oil supplied into the reception port from the oil source is supplied through the main function relay port (220P, 260P, 265P, 270P, 275P) to the main function hydraulic actuator (20, 50, 60) that is operatively controlled by the valve accommodated in the main function valve block (200 -200); and the additional function valve block (300) being mountable on the second region so as to realize a state in which hydraulic oil is supplied through the removal port, a communication oil passage formed in the housing, the additional function reception port and the additional function relay port (330P, 335P, 340P, 345P) to the additional function hydraulic actuator (70, 80) that is operatively controlled by the valve accommodated in the additional function valve block (300).