A blood sensor comprises a substrate, a spacer attached to the top of the substrate, a cover attached to the top of the spacer. A blood reservoir defined by a substrate hole formed in the substrate. A part of a spacer hole in the spacer and connected to the substrate hole, and a cover hole formed in the cover is connected to the spacer hole. A supply passage is defined by another part of the spacer hole that communicates with the blood reservoir portion, and detection electrodes are formed in the supply passage. The cover projects from the supply passage toward the inside of the blood reservoir portion farther than the substrate and the spacer.