Multiple grooves are formed along an inclination face of a prism-like part. A pitch of the grooves is smaller than the wavelength of light, specifically infrared light, used as a light source for detecting an ink amount. When sufficient amount of contained ink is consumed to expose the prism-like part, ink on the inclination face is held in the grooves due to their capillary force. At this time, if an amount of the ink exceeds the capacities of the grooves, no capillary force acts on the excessive ink. Accordingly, the excessive ink becomes an ink pool and slides down to an ink surface. Since the face provided with the grooves at the pitch smaller than the wavelength of the detection light source serves as a plane face optically, detection with high reliability is made possible.