When a sample of biological origin in an aqueous solution is used as the measurement medium in analysis using an electrochemical process, and a voltage of +1.2 V or greater (with saturated silver-silver chloride electrode potential as a reference) is applied, there are instances in which bubbles are observed to be produced within the flow cell, due to an electrolysis reaction deriving from the measurement buffer. There is a possibility that bubbles produced on the electrode will cover the electrode surface, reducing the effective surface area of the electrode. Also, the distribution of magnetic particles captured on the electrode will be disturbed by the gas produced thereby, lowering the reproducibility of the results of the analysis. Deaeration of the measurement medium prior to introduction of the measurement medium into the detector minimizes the effects of bubble production in degrading the analytical capability makes it possible to carry out highly sensitive electrochemical analysis.