The purpose of this study was to suggest the use of laser energy in the the field of operative dentistry without considerable pulpal damage and significant effects on the dental hard tissue, additionally to find out the methods which could control the temperature rise. The laser beam (CW $CO_2$ laser, output: 6W, beam diameter: 1.5mm) was focused on the center of the occlusal surface of extracted lower molars. A Ge lens (focal length 200mm) was used to focus the primary laser beam. In order to vary the total amount of the same irradiated energy, experimental subjects were devided into three groups: continuously irradiated group, intermittently irradiated group, and water-cooled group after continuous laser irradiation. Temperature changes in the pulp chamber after laser irradiation were measured and recorded by the digital thermometer and recorder. The following results were obtained: 1. Temperatures in the pulp chamber were raised up in the order of the continuously irradiated group, intermittently irradiated group, water-cooled group after continuous laser irradiation. 2. In the continuously irradiated group, the temperature was raised up $1.7^{circ}C$, $3.8^{circ}C$, $7.3^{circ}C$, $17.2^{circ}C$ after 2, 4, 8, 16 seconds of the irradiation of laser. In the intermittently irradiated group, the changes were $1.2^{circ}C$, $3.4^{circ}C$, $6.3^{circ}C$, $11.1^{circ}C$, respectively. In the water-cooled group after continuous laser irradiation, the changes were $0.0^{circ}C$, $0.8^{circ}C$, $1.6^{circ}C$, $6.9^{circ}C$, respectively. 3. The starting time of temperature rise in the pulp chamber had no connection with laser irradiation time.