An object of the present invention is to provide a method of quantitative determination of pyrophosphate (PPi) and a method of detecting a primer extension reaction that require fewer kinds of enzymes and that do not necessitate strict temperature regulation, and a kit and a device for performing these methods. In the method of measuring PPi of the present invention, a PPi sample having an unknown concentration is allowed to act on mycobacterial H+-pyrophosphatase (H+-PPase) intrinsically included in membrane, thereby measuring the PPi concentration in the sample by analyzing thus resulting H+transport. Furthermore, in the method of detecting an extension reaction of a primer of the present invention, an unknown nucleic acid sample to which examination of occurrence of the primer extension reaction is intended is allowed to act on mycobacterial H+-PPase intrinsically included in a membrane, thereby determining occurrence of the primer extension reaction on the unknown nucleic acid sample by analyzing thus resulting H+transport.