A sensor including an insulating base plate; an electrode system, provided on the base plate, having a measuring electrode and a counter electrode; a reaction layer including at least an oxidoreductase and an electron mediator; a sample solution supply pathway including the electrode system and the reaction layer; and a sample supply unit, wherein the sensor is so structured that, between the sample supply unit and the sample solution supply pathway, there is provided a filter having a function to filter out hemocytes and having a cross-sectional area larger than an opening of the sample solution supply pathway, and that plasma of a blood with hemocytes thereof having been filtered out is sucked into the inside of the sample solution supply pathway owing to capillary phenomena.