A cutting tool holder unit is composed of a cutting tool holder adapted to detachably mount a cutting tool thereon, and a front cover of a flange shape slidably fitted into a side of the cutting took holder mounted on a multi-spindle head. A groove formed on the inner circumferential surface of the front cover is slidably fitted into a groove formed on the cutting tool holder to form a cutting oil circulating channel and a cutting oil injection port is opened at an upstream position of a cutting tool mounting section relative to the direction of rotation of the cutting tool holder.