We have been proposing an information gathering method using a mobile robot-assisted sensor network system to support mitigation activity in post-disaster area. Our method includes gathering of information in closed area where the mobile robot cannot enter by launching deployment of a sensor node. The paper describes a mechanism for controlling a cable length that is released by launching of wired sensor node. The cable is wound on a reel that a powder brake and a rotary encoder are mounted on its rotation shaft. The cable length is measured by a rotary encoder, and is adjusted by breaking the rotation of the shaft by powder brake. In the evaluation of the mechanism, we measured the length of cable connecting sensor node by free fall experiment and launching experiment. Then, an experiment confirmed that the proposed mechanism could measure the length of a cable and control the wired sensor node.