A crawler track tension adjusting device is provided in which a driving unit having a closed structure carries out automatic tension adjustment in response to a control signal to create optimum tension on a crawler belt. The crawler track tension adjusting device includes a tension adjusting cylinder which is operable to control the tension and is actuated by a hydraulic pump that is driven by a motor. An electromagnetic direction selector valve is disposed in an oil pipeline for connecting the hydraulic pump to the tension adjusting cylinder, and a hydraulic sensor for detecting the operating condition of the tension adjusting cylinder is disposed in the hydraulic circuit in which the tension adjusting cylinder, the hydraulic pump and the selector valve are provided. The hydraulic sensor is connected to a controller to operate the hydraulic pump and the direction selector valve so as to control the tension adjusting cylinder.