Each scanning optical system includes a first scanning lens and a second scanning lens through which a light beam is incident onto the first scanning lens. The spacer includes a body defining a longitudinal passageway. When the sensor has detected an absence of ink in which ink level is below the predetermined level and it is judged that the sensor has detect a presence of ink, the value of the counter is made a threshold value corresponding to a level at which the sensor detects the absence of ink and then this threshold value is used to determine the remaining ink level. The adaptive combiner forms a weighted summation of the spatially unique replicas, which effectively places spatial nulls in the direction of the multipath interference.