An information processing device is provided with a calculating unit and a determining unit. The calculating unit calculates, as a length calculation value, a length between parts set for measuring the length of the object, from an image of the object in a captured image in which the object to be measured has been imaged. In accordance with a pre-assigned selection rule, the determining unit selects, from among length calculation values calculated respectively from a plurality of captured images having different image capture time points within a set time range, the length calculation value when the object is in a basic attitude for length measurement, and an assumed length calculation value, and determines the measured value of the length of the object using the selected length calculation values.