A failure diagnostic apparatus for use in a ball screw conveyer includes a bearing stopper placed in contact with a bearing supporting a ball screw, a sensor unit equipped with an elastic member and a heat flux sensor, and a malfunction detector. The heat flux sensor works to produce an output as a function of deformation or displacement of the bearing stopper. The malfunction detector works to analyze the output from the heat flux sensor to detect a failure in operation of the ball screw conveyer. This enables a malfunction of the ball screw conveyer to be detected quickly and accurately.