In a master cylinder device for a vehicle having a master cylinder with a cylinder body attached in the vicinity of a grip of a steering bar handle, and an operating lever which is supported on the cylinder body through a pivot and is pulled toward the grip to thereby push a piston of the master cylinder, the cylinder body is mounted in such a position that, below the axis of the bar handle, the axis of the cylinder body is substantially orthogonal to a vertical plane which includes the axis of the bar handle, whereby the master cylinder can be mounted even in a narrow space on the bar handle in a motorcycle and the like having a steering bar handle.