This paper deals with the removal characteristics of dental hard tissues by irradiation of a pulsed Er:YAG laser beam with a wavelength of 2940 nm. The influence of water-layer thickness on the cavity preparation is experimentally investigated. As a specimen, extracted human tooth is used and the laser beam is irradiated to its surface through the optical fiber. Additionally, the phenomenon of the laser beam irradiation in the water environment is observed with a high speed video camera. As a result, the irradiated laser beam in water environment is immediately absorbed to water, and the evaporated area could make it possible to transmit the laser beam without any absorption. The removal volume on human tooth in water environment was increased with increasing the irradiated laser energy, and decreased with increasing the water-layer thickness. There were no thermal damages on the cavity surface in each condition. The eliminated volume in water environment was quite larger than that in air environment. The human tooth in water environment was eliminated by the bubble formation which was caused by laser beam irradiation in addition to the ablation of human tooth which was caused by the absorption of laser beam directly.