There is provided a cholesterol sensor with high-accuracy and excellent response, whose object to be measured is whole blood, where plasma with which is obtained by filtering out hemocytes therein filtered in blood can rapidly reach an electrode system. In a biosensor where plasma with which is obtained by filtering out hemocytes therein filtered with by a filter is sucked into a sample solution supply pathway due to capillarity, there are formed: a first pressing part for holding a primary side portion of the filter from the bottom; a second pressing part for holding a secondary side portion of the filter from the top and the bottom; a third pressing part for holding the central portion of the filter from the top; and a void for surrounding the filter between the second pressing part and third pressing part.