A portable brush cutter comprising: a cutting blade rotating about a rotating shaft; and a transmission shaft positioned between an output shaft of a prime mover and the rotating shaft, the transmission shaft being adapted to couple the output shaft and the rotating shaft together when they are driven, rotation of the output shaft of the prime mover being transmitted to the cutting blade via a rotation transmission path, wherein: a one-way clutch to prevent generation of torsional vibration in the rotation transmission path is interposed between the output shaft and the transmission shaft or between the rotating shaft and the cutting blade, wherein the prime mover includes a clutch for the prime mover adapted to transmit the rotation of the output shaft to the transmission shaft, wherein the clutch for the prime mover includes a clutch drum provided on a side of the transmission shaft, wherein the clutch drum includes a receiver extending forward from a center of rotation of the clutch drum, wherein a rear end of the transmission shaft is received in the receiver, characterized in that bearings (16,17) are adjacently mounted on the opposite sides of the one-way clutch, wherein the one-way clutch and the bearings (16,17) are interposed between the clutch drum and the transmission shaft, wherein the one-way clutch and the bearings (16,17) are fit-inserted and disposed between the receiver and the rear end of the transmission shaft with no clearance, and wherein a portion of the receiver coupled to the clutch drum is closed. a cutting blade rotating about a rotating shaft; and a transmission shaft positioned between an output shaft of a prime mover and the rotating shaft, the transmission shaft being adapted to couple the output shaft and the rotating shaft together when they are driven, rotation of the output shaft of the prime mover being transmitted to the cutting blade via a rotation transmission path, wherein: a one-way clutch to prevent generation of torsional vibration in the rotation transmission path is interposed between the output shaft and the transmission shaft or between the rotating shaft and the cutting blade, wherein the prime mover includes a clutch for the prime mover adapted to transmit the rotation of the output shaft to the transmission shaft, wherein the clutch for the prime mover includes a clutch drum provided on a side of the transmission shaft, wherein the clutch drum includes a receiver extending forward from a center of rotation of the clutch drum, wherein a rear end of the transmission shaft is received in the receiver, characterized in that bearings (16,17) are adjacently mounted on the opposite sides of the one-way clutch, wherein the one-way clutch and the bearings (16,17) are interposed between the clutch drum and the transmission shaft, wherein the one-way clutch and the bearings (16,17) are fit-inserted and disposed between the receiver and the rear end of the transmission shaft with no clearance, and wherein a portion of the receiver coupled to the clutch drum is closed. a one-way clutch to prevent generation of torsional vibration in the rotation transmission path is interposed between the output shaft and the transmission shaft or between the rotating shaft and the cutting blade, wherein the prime mover includes a clutch for the prime mover adapted to transmit the rotation of the output shaft to the transmission shaft, wherein the clutch for the prime mover includes a clutch drum provided on a side of the transmission shaft, wherein the clutch drum includes a receiver extending forward from a center of rotation of the clutch drum, wherein a rear end of the transmission shaft is received in the receiver, characterized in that bearings (16,17) are adjacently mounted on the opposite sides of the one-way clutch, wherein the one-way clutch and the bearings (16,17) are interposed between the clutch drum and the transmission shaft, wherein the one-way clutch and the bearings (16,17) are fit-inserted and disposed between the receiver and the rear end of the transmission shaft with no clearance, and wherein a portion of the receiver coupled to the clutch drum is closed.