There is disclosed means for quickly solving instability of sensor sensitivity performances found in an initial stage and stabilizing the sensor sensitivity performances, when immersing a chemical sensor kept under a dry state in a buffer solution used as a storage liquid and applying a measurement bias between a working electrode and a reference electrode to make first use of the chemical sensor for measurement in which the chemical sensor is used. To make the first use of the chemical sensor, after immersing the chemical sensor kept under a dry state in the buffer solution used as the storage liquid, a first initial treatment bias having the same direction as that of the measurement bias and possessing an absolute value larger than that of the measurement bias is applied between the working electrode and the reference electrode for a first initial treatment time. Subsequently, the bias is changed to a second initial treatment bias which is the same as the measurement bias, and the second initial treatment bias is applied for a second initial treatment time. When such a two-step initial treatment operation is carried out, the stabilized sensor sensitivity performance is achieved.