Recently, fire detection systems are widely used to protect life and goods. However, conventional fire detecting systems cannot prevent fire actually. In this paper we propose a new method to predict fire occurrence time at early stage in order to prevent failing to get out and losing aged person's life in a fire. We used odor, air and CO sensors to develop fire detection system, in order to generate alarms as early as possible. Based on the derivative value of sensors output, we used linear approximation method to predict the time that the sensor outputs reach smoldering fire situation and fire situation. Therefore we can generate pre-alarm before these situations. In the future, we also want to use them to monitor the room environment.