There is provided a linear displacement measuring apparatus which notifies a user when being installed defectively. A slider includes a traveling body which travels along a main scale on the main scale, a carriage part mounted on an object, and coupling means which couples the traveling body with the carriage part. The coupling means allows relative displacement between the traveling body and the carriage part and biases the traveling body towards the main scale. The coupling means has a connecting rod arranged so as to couple the traveling body with the carriage part via a ball joint. The slider further has misalignment detecting means which detects that the relative displacement between the traveling body and the carriage part exceeds a prescribed allowable range. The misalignment detecting means is a pair of projecting pieces arranged in a vicinity of the connecting rod so that the displacement of the connecting rod is regulated within a predetermined range.