  It has been a puzzle that rotating detector may respond even in the
appropriate vacuum defined via canonical quantization. We solve this puzzle by
taking back reaction of the detector into account. The influence of the back
reaction, even in the detector's mass infinite limit, appears in the response
function. It makes the detector possible to respond in the vacuum if the
detector is rotating, though the detector in linear uniform motion never
respond in the vacuum as expected from Poincare invariance.
