An apparatus for monitoring the operation of a robot joint in which a second member (43) is displaceably mounted on a first member (41). The distance to a measurement plate provided to the second member (43) is measured by a range finder (42) provided to the first member (41). A monitoring unit (71) compares the distance data measured by the range finder with a reference distance stored in advance, and generates an abnormality signal when the difference between the distance data and the reference distance exceeds an allowable value.