A blood sensor (20a) comprises a substrate (21)； a spacer (22) attached to the top of the substrate (21)； a cover (23) attached to the top of the spacer (22)； a blood reservoir portion (24) defined by a substrate hole (21a) formed in the substrate (21), a part of a spacer hole (22a) formed in the spacer (22) and connected to the substrate hole (21a), and a cover hole (23a) formed in the cover (23) and connected to the spacer hole (22a)； a supply passage (25) defined by another part of the spacer hole (22a) and communicating with the blood reservoir portion (24)； and detection electrodes formed in the supply passage (25). The cover (23) projects from the supply passage (25) toward the inside of the blood reservoir portion (24) farther than the substrate (21) and the spacer (22).
