A linear guideway is characterized in that it is capable of determining whether there is abnormal displacement of the return member based on the open or close of the circuit, and can output the abnormal signal immediately when the circuit is open. Compared with the conventional structure, since the present invention can be disposed only at one end of the slide block, both the number of components and assembly time can be reduced, and the diagnosis with precision and immediacy can be performed without high-deep algorithm, which can play a role of instant reminding before the linear guideway completely loses function. Therefore, the service life of the linear guideway can be known early to avoid the loss caused by the shutdown.