Disclosed are a novel means capable of accurately calculating the amount of in-vivo components as a test substance, and its uses.A sensor assembly 20 includes an electrochemical sensor 30 for measuring a test substance percutaneously extracted from a living body, and a processor 115a for calculating the intensity of the background signal at the measurement time point of the test substance after the elapse of the predetermined time based on the change over time of the intensity of the background signal measured within a predetermined time before measurement of the test substance by the electrochemical sensor 30. In this way, the amount of the in-vivo component as the test substance can be accurately calculated since the calculated value of the intensity of the background signal to be subtracted from the unprocessed signal including the signal attributable to the test substance can be obtained.
