The present invention relates to a technique for measuring a glucose level by utilizing a reaction system containing an enzyme and an electron carrier. In accordance with the glucose level measuring method of the present invention, glucose dehydrogenase with cytochrome C attached thereto or glucose dehydrogenase derived from a microorganism belonging to a burkholderia genus is used as the enzyme, and a Ru compound is used as the electron carrier. The present invention further provides a glucose sensor in which glucose dehydrogenase with cytochrome C attached thereto or glucose dehydrogenase derived from derived from a microorganism belonging to a burkholderia genus is used as the enzyme, and a Ru compound is used as the electron carrier.