More specifically, in the cross-reference table TB, “Y” representing the yellow ink flow path, “T1” representing the light emitter T1, “Yellow” representing the color of the light emitted from the light emitter T1, “R1” representing the light receiver R1, and “Yellow” representing the allocated color, are associated with one another, “C” representing the cyan ink flow path, “T2” representing the light emitter T2, “Blue” representing the color of the light emitted from the light emitter T2, “R2” representing the light receiver R2, and “Blue” representing the allocated color, are associated with one another, “M” representing the magenta ink flow path, “T3” representing the light emitter T3, “Red” representing the color of the light emitted from the light emitter T3, “R3” representing the light receiver R3, and “Red” representing the allocated color, are associated with one another, and “Bk” representing the black ink flow path, “T4” representing the light emitter T4, “Purple” representing the color of the light emitted from the light emitter T4, “R4” representing the light receiver R4, and “Purple” representing the allocated color, are associated with one another.