A detection unit to optically detect a discharge condition of a droplet is disclosed. The detection unit includes a light emitting element and a light receiving element disposed on opposite sides of an area through which a droplet discharged from a droplet-discharger passes. A diaphragm plate having an aperture and another diaphragm plate having at least two apertures arranged at a pitch in a discharge direction are respectively disposed near front surfaces of the two elements. When light emitted from the light emitting element passes through the apertures, two light beams are received by the light receiving element. When the droplet is discharged and passes in front of the apertures, the two light beams are blocked sequentially by the ink droplet, which causes the quantity of light received by the light receiving element to change, thereby inducing a change in an output from the light receiving element. Based on the change in the output, a discharge-condition of the droplet is determined.