This blood flow sensor 1 can measure the flow speed of blood B, and is provided with a tube 40 which defines a flow path 42 through which the blood B flows, a light-emitting element 10 which emits a laser beam L, and a light-receiving element 20. The light-emitting element 10 emits a laser beam L towards the blood B flowing through the tube 40. The light-receiving element 20 receives non-Doppler-shifted light which is emitted by the light-emitting element 10 and reflected by the tube 4, and Doppler-shifted light which is emitted by light-emitting element 10 and reflected by a blood component flowing through the tube 40. Defining a first direction and a second direction as two orthogonal directions in a cross-section perpendicular to the flow direction of the blood B, the width w1 of the flow path in a first direction is less than the width w2 in the second direction. The light-emitting element 10 and the light-receiving element 20 are arranged on the outer surface of the tube wall that defines the width w1 in the first direction.
