To measure the preparation range of an improved body with high reliability without conducting experiment works.

The measuring system 1 of the prepared improved body has a casing 3, in which an injection pipe 2 can be inserted into a hollow section, annular electrodes 41, 42,...4n installed onto the external surface of the casing along the axis of a member of the casing, a Dc power 6 and a voltmeter 7 electrically connected to the electrodes through a changeover instrument 5 as a changeover means and an arithmetic and control section 8 controlling these each equipment. The changeover instrument 5 can select desired electrodes in the electrodes 41, 42,...4n as current electrodes 4a, 4b and potential electrodes 4c, 4d. Electricity is conducted through the improved body 11 prepared around the casing 3 by operating the DC power 6 and applying voltage between the current electrodes 4a, 4b by the arithmetic and control section 8 while potential difference between the potential electrodes 4c, 4d is measured and the prepared range of the improved body 11 is evaluated from the relationship of potential difference and a conducted current value.