This paper presents the feasibility of a sensor system consisting of piezoelectric oscillator sensors to detect local damages and ultrasonic sensors to detect global damages in a structure. The oscillator sensor is composed of a feedback oscillator circuit and a piezoceramic lateral mode vibrator to be attached to a structure. Damage to the structure causes a change in the resonant frequency of the vibrator, which is detected by the oscillator circuit. However, the response of the oscillator sensor is limited to the area around the sensor, thus local measurement. The ultrasonic sensor generates Lamb waves and the waves traveled over a long distance are received by another piezoceramic patch on the structure. The received wave form reflects all the defects encountered during the propagation, thus global measurement is possible. The two sensor types are combined as a sensor network, and its operation system is developed as a portable unit for practical applicability.