Tooth decay is a microbial infectious disease that caused to local damage of calcification tissues of tooth and common cause of tooth damage in youth. Therefore, prevention of this damage is necessary. One of the most important methods of prevention of tooth decay is opportune diagnosis of decay and reconstruction. The present study aimed to introduce a novel invention for early diagnosis of tooth decay and to compare result of experiment of this device with other common methods. This device will diagnose tooth decay without any chemicals and rays such as lasers or X-rays. It works by shining infrared light reflected on the surface of the tooth. It will be placed on the tooth and Transmitting infrared light will be reflected on the surface of the tooth and Infrared sensor receives light from the tooth surface. All lights are reflected, but if the tooth was rotten, then its color would be darker and the reflected light would be absorbed on the surface of the teeth. The rest of the light is reflected. By the difference between the amount of light reflected from the surface of decayed teeth and Healthy Teeth the device is able to detect decay and dental health. Infrared sensor is a type of optical transistors. When the light is shone on them, the light will be converted into an electric current. This electric current will be changed in different expressions. The microcontroller device will process these flows. The phase logic is used for data processing that substantially increases its accuracy. The invented device does not use chemicals to detect caries which is quite safe and affordable. Comparison the result of diagnosis of damaged tooth using this device with diagnoses of two independent dentists indicated more accuracy, efficiency and confidential (p<0.01). The invented device can be used as the primary method of dental caries.