Vibration mode of a truss structure is different to that of a cantilever. The objective of the present work is to establish a new sensing technique for specific vibration mode of the truss. In this method the distributed modal sensor is employed to determine the vibration mode. First the design process for a modal filtering method is examined using a one-dimensionally distributed modal sensor. Then the shape of the vibration mode, which is required for designing the sensor, is determined and, based on the displacement distribution of this mode shape, a modal filter can be designed. If the sensor is designed under the condition of a fixed vibration mode, the error signals introduced during the measurement are considered to cause unfavorable effect on the sensitivity. As a counter measure, a polynomial approximation with minimum errors is used to smooth the measured vibration mode. The modal filter designed above is attached to the truss structure and excitation tests of the truss are conducted for evaluating separation performance of the modal filter. By comparing the results of the present system with a distributed modal sensor and the inertial mass actuator system commonly used so far, it is found that the present sensing method is far more useful.