Abstract: A work machine in which a base end section of an arm is pivotally supported by a tip end section of a boom in the front-and-rear direction in an oscillating manner. The work machine includes an arm angle sensor apparatus for detecting an oscillating angle of the arm so that the arm angle sensor apparatus is protected and an increase in cost is suppressed. A sensor body of the arm angle sensor apparatus is attached to a sensor attachment hole provided in the third boom side plate such that the sensor attachment hole includes a part of the sensor body. The link member for transmitting the oscillating angle of the arm to the sensor body is provided between the third boom side plate and the arm side plate such that one end side is connected to the sensor body and the other end side is connected to the arm side plate.