PURPOSE: To lead out and count time wherein γ-Fe2O3 is left as it is after the time of final heating hysteresis thereof, by using a dynamic behavior of an electric resistance value when gamma-type Fe2O3 is heated by a heater.

CONSTITUTION: γ-Fe2O3, a heater for heating the same, and an electrode to detect the behavior of an electric resistance value are provided. An element prepared by heating once γ-Fe2O3 and subjecting the same to gas desorption shows once a PTC characteristic having a peak at a temperature of 100°C, as shown in the figure (a), after it is heated by the heater in the case when a time of being left as it is subsequently to the former treatment, and afterward, it settles down to a resistance value in the state of equilibration at the heating temperature of the heater. To the contrary, in the case when the time of being left is long, the element shows the behavior of an NTC characteristic as shown in the figure (b) and, in addition, overshoots once to a very low value of the resistance value. Thereafter, it becomes stable also at the resistance value in the state of equilibration resistance value of γ-Fe2O3 at this time is detected and the time of being left is measured.