State monitoring device for speed reducer and state monitoring method for speed reducer

One purpose of the present invention is to provide a state monitoring device capable of improving the accuracy of monitoring. A state monitoring device (10) according to an embodiment of the present invention is a state monitoring device for a speed reducer (8) mounted between a first arm (1) and a second arm (2) of a robot hand (100), the state monitoring device being provided with: a measurement mechanism (4) for measuring the relative position of the first arm (1) and the second arm (2); and a monitoring unit (5) that monitors the amount of change in the relative position measured by the measurement mechanism (4) with respect to the reference value in the predetermined specific posture. 本发明的目的之一在于提供一种能够改善监视的精确度的状态监视装置。一种实施方式的状态监视装置(10)为搭载于机械手(100)的第1臂(1)与第2臂(2)之间的减速机(8)的状态监视装置，该状态监视装置具备：测量机构(4)，测量第1臂(1)及第2臂(2)的相对位置；及监视部(5)，监视在规定的特定姿势下由测量机构(4)测量到的相对位置相对于基准值的变化量。