To explore a fantasy world experienced sometime and somewhere in our memory. Phantasm is an interactive installation in which participants light up in a small dreamlike world in space. As you take hold of a glowing sphere that releases a pale light, the area lights up in pale blue light, and white butterflies appear from nowhere as a soft piano melody flows. Butterflies fly slowly and gloriously, gathering toward the sphere, and chase you as you move the sphere. The sphere is the key to the real world and fantasy. Participants can experience the nostalgia of playing with butterflies by moving around or holding the sphere in the air. Beautiful white butterflies draw wing strokes in the air as if they are symbols of a fantasy world; they delicately lead you into their world. If you cover the sphere with your hands to shut out the light, the butterflies gradually disappear, leaving silence and lingering light, bringing you back to reality. You will find yourself perplexed as if it were a fleeting dream. Technical statement of “Phantasm” LEDs are used as the light source for the sphere which is the interface, so that the sphere glows in uniformity and allows the interaction of participants hiding the light or releasing the light with their hands. Light, color tone, and the position of the sphere are monitored with a sensor camera placed on the ceiling, and by connecting with real time CG, the butterfly movements (appearance, disappearance, gathering, and chasing) become possible.