PURPOSE: To remove the influence due to conditions having no relation with unevenness such as temperature by referring only the difference in change of material and the like in an uneven portion, and detect the uneven portion by determining the change in difference or ratio in the respective parts in liquid.

CONSTITUTION: By an electric power source from an oscillator 1 the conductvity is determined in an upper electrode portion 2 and a lower electrode portion 3, the signal 4 and difference 5 of the measured value are detected and amplified by a differential amplifier 6, a reference value is set by a setting device 7, compared by a comparator 8 and in the case of the reference value being more than a reference set value, an alarm device 9 is actuated.