A power transmission device for vehicles comprising: a casing that is mounted on a vehicle; a transmission that is provided in said casing to change rotational speeds of an engine; a torque converter that is mounted in said casing to transmit the rotation of said engine through fluid to said transmission; and a lockup device that is mounted in said torque converter to transmit the rotation of said engine directly to said transmission, wherein said torque converter includes a rotating body that is rotatably supported in said casing and is rotated by said engine, and a hollow cylindrical transmission shaft that is rotatably supported in said casing to transmit the rotation of said rotating body through fluid or said lockup device to said transmission, and said lockup device includes a lockup clutch for establishing connection between said rotating body and said transmission shaft by pressurized oil delivered from a hydraulic pump, a lockup control valve that controls supply/discharge of the pressurized oil to/from said lockup clutch, and a lockup oil passage that introduces the pressurized oil from said lockup control valve to said lockup clutch, characterized in that: said lockup control valve is positioned on an outer side face of said casing and is arranged in a position of overlapping said transmission shaft in a radial direction of said transmission shaft; said lockup oil passage includes a casing side oil passage that is provided in said casing to be connected to said lockup control valve, a transmission shaft side oil passage (77, 82) that is provided in said transmission shaft and an inlet side of which is connected to said casing side oil passage, and a rotating body side oil passage that is provided in said rotating body for establishing connection between an outlet side of said transmission shaft side oil passage (77, 82) and said lockup clutch; and said casing side oil passage is formed as a linear oil passage that linearly extends in the radial direction of said transmission shaft between said lockup control valve and said transmission shaft side oil passage (77, 82), characterized in that a surface portion of said outer side surface in said casing, on which said lockup control valve is mounted, is formed as a concave recess with a flat valve mounting surface recessed from said outer side surface in said casing toward said transmission shaft. a casing that is mounted on a vehicle; a transmission that is provided in said casing to change rotational speeds of an engine; a torque converter that is mounted in said casing to transmit the rotation of said engine through fluid to said transmission; and a lockup device that is mounted in said torque converter to transmit the rotation of said engine directly to said transmission, wherein said torque converter includes a rotating body that is rotatably supported in said casing and is rotated by said engine, and a hollow cylindrical transmission shaft that is rotatably supported in said casing to transmit the rotation of said rotating body through fluid or said lockup device to said transmission, and said lockup device includes a lockup clutch for establishing connection between said rotating body and said transmission shaft by pressurized oil delivered from a hydraulic pump, a lockup control valve that controls supply/discharge of the pressurized oil to/from said lockup clutch, and a lockup oil passage that introduces the pressurized oil from said lockup control valve to said lockup clutch, characterized in that: said lockup control valve is positioned on an outer side face of said casing and is arranged in a position of overlapping said transmission shaft in a radial direction of said transmission shaft; said lockup oil passage includes a casing side oil passage that is provided in said casing to be connected to said lockup control valve, a transmission shaft side oil passage (77, 82) that is provided in said transmission shaft and an inlet side of which is connected to said casing side oil passage, and a rotating body side oil passage that is provided in said rotating body for establishing connection between an outlet side of said transmission shaft side oil passage (77, 82) and said lockup clutch; and said casing side oil passage is formed as a linear oil passage that linearly extends in the radial direction of said transmission shaft between said lockup control valve and said transmission shaft side oil passage (77, 82), characterized in that a surface portion of said outer side surface in said casing, on which said lockup control valve is mounted, is formed as a concave recess with a flat valve mounting surface recessed from said outer side surface in said casing toward said transmission shaft. said lockup control valve is positioned on an outer side face of said casing and is arranged in a position of overlapping said transmission shaft in a radial direction of said transmission shaft; said lockup oil passage includes a casing side oil passage that is provided in said casing to be connected to said lockup control valve, a transmission shaft side oil passage (77, 82) that is provided in said transmission shaft and an inlet side of which is connected to said casing side oil passage, and a rotating body side oil passage that is provided in said rotating body for establishing connection between an outlet side of said transmission shaft side oil passage (77, 82) and said lockup clutch; and said casing side oil passage is formed as a linear oil passage that linearly extends in the radial direction of said transmission shaft between said lockup control valve and said transmission shaft side oil passage (77, 82),