An analyzer (A) includes a current measurer (42) for measuring the current flowing between paired electrodes (22a), (22b) when a sample is introduced to the reagent layer (23) of a sensor (2) and voltage is applied across the electrodes, while also including a processor (3) for analyzing the sample based on the current after a predetermined reference time point. The analyzer (A) further includes a fail determiner (44) which monitors the change of the current before and after the reference time point and determines that the sensor (2) is improper when the current change does not correspond to a predetermined current change.