Efficiency of a charging processing of an insulator sample is improved. Thereafter, the precharged state of the bit lines is canceled, and the gate level of each charge transfer transistor is raised to an appropriate value while the sense amplifier nodes are maintained in the precharged state, thereby copying the threshold voltage difference between the charge transfer transistors as the potential difference between the pair of bit lines. The selected memory cell operates as a source follower to set a voltage on the coupled bit line at the read gate voltage minus the threshold voltage of the cell, allowing the voltage of the cell to be directly sensed or sampled. These trustworthy probe traces are tracked during travel on other road segments whose attribute data is unknown or unreliable.