We developed a new thermometer that uses color of volcanic ashes. A series of heating experiments for basaltic ashes under the atmospheric condition provided a quantitative relationship among color, heating temperature, and heating duration. The higher the heating temperature, the more the redness in color of heated ash. We applied the relationship to estimate heated temperature of the ash that was underlying below or contact with a cauliflower-shaped volcanic bomb ejected from the Miyakejima volcano on 18 August, 2000. The estimated temperature was about 390℃ for the ash underlying 1 cm below the volcanic bomb, and 550℃ for the ash in contact with the bomb. Numerical heat transfer calculations for the volcanic bomb on the ash layer suggested that temperature of its center at the time of landing is about 1,000℃. This is the first concrete evidence that the bomb was essential material and that the 18 August eruption was phreatomagmatic.