The object of the present invention is to provide a substance, which is easy to handle and enables the measurement of ATP with a high sensitivity regardless of the concentration of protein, and further a measuring method of ATP using the substance. Such object is solved with a fluorescence labelled fusion protein obtained by attaching two types of fluorescent substances of potential donor and acceptor for fluorescence resonance energy transfer (FRET) respectively to a protein which can cause structural changes depending on ATP binding, namely ε protein, which is the subunit of ATP synthetase, and further solved by contacting the fluorescence labelled fusion protein with a subject substance and then measuring the fluorescence spectra.