A method for acquiring gap information related to a gap between an ink discharging surface of an inkjet printer and a recording sheet, the inkjet printer comprising: an inkjet head configured to discharge ink droplets from nozzles formed in the ink discharging surface thereof; a head scanning unit configured to reciprocate the inkjet head relative to the recording sheet along a scanning direction parallel to the ink discharging surface; and a wave shape generating mechanism (17, 18) configured to deform the recording sheet in a predetermined wave shape having tops (Pt) of portions (Pm) protruding in a first direction toward the ink discharging surface and bottoms (Pb) of portions (Pv) recessed in a second direction opposite to the first direction, the tops (Pt) and the bottoms (Pb) alternately arranged along the scanning direction, the method comprising steps of: acquiring gap information related to a gap between the ink discharging surface and each individual one of the tops (Pt) and the bottoms on the recording sheet; and determining whether the gap information acquired for each individual one of the tops (Pt) and the bottoms (Pb) on the recording sheet is abnormal, the determining step characterised by comprising the steps of: determining a first representative value for all pieces of the gap information acquired for the tops (Pt); when a deviation of gap information acquired for a top (Pt) relative to the first representative value is equal to or more than a predetermined first threshold, determining that the gap information acquired for the top (Pt) is abnormal; determining a second representative value; and when a deviation of gap information acquired for a bottom (Pb) relative to the second representative value is equal to or more than a predetermined second threshold, determining that the gap information acquired for the bottom (Pb) is abnormal. an inkjet head configured to discharge ink droplets from nozzles formed in the ink discharging surface thereof; a head scanning unit configured to reciprocate the inkjet head relative to the recording sheet along a scanning direction parallel to the ink discharging surface; and a wave shape generating mechanism (17, 18) configured to deform the recording sheet in a predetermined wave shape having tops (Pt) of portions (Pm) protruding in a first direction toward the ink discharging surface and bottoms (Pb) of portions (Pv) recessed in a second direction opposite to the first direction, the tops (Pt) and the bottoms (Pb) alternately arranged along the scanning direction, the method comprising steps of: acquiring gap information related to a gap between the ink discharging surface and each individual one of the tops (Pt) and the bottoms on the recording sheet; and determining whether the gap information acquired for each individual one of the tops (Pt) and the bottoms (Pb) on the recording sheet is abnormal, the determining step characterised by comprising the steps of: determining a first representative value for all pieces of the gap information acquired for the tops (Pt); when a deviation of gap information acquired for a top (Pt) relative to the first representative value is equal to or more than a predetermined first threshold, determining that the gap information acquired for the top (Pt) is abnormal; determining a second representative value; and when a deviation of gap information acquired for a bottom (Pb) relative to the second representative value is equal to or more than a predetermined second threshold, determining that the gap information acquired for the bottom (Pb) is abnormal. acquiring gap information related to a gap between the ink discharging surface and each individual one of the tops (Pt) and the bottoms on the recording sheet; and determining whether the gap information acquired for each individual one of the tops (Pt) and the bottoms (Pb) on the recording sheet is abnormal, the determining step characterised by comprising the steps of: determining a first representative value for all pieces of the gap information acquired for the tops (Pt); when a deviation of gap information acquired for a top (Pt) relative to the first representative value is equal to or more than a predetermined first threshold, determining that the gap information acquired for the top (Pt) is abnormal; determining a second representative value; and when a deviation of gap information acquired for a bottom (Pb) relative to the second representative value is equal to or more than a predetermined second threshold, determining that the gap information acquired for the bottom (Pb) is abnormal. determining a first representative value for all pieces of the gap information acquired for the tops (Pt); when a deviation of gap information acquired for a top (Pt) relative to the first representative value is equal to or more than a predetermined first threshold, determining that the gap information acquired for the top (Pt) is abnormal; determining a second representative value; and when a deviation of gap information acquired for a bottom (Pb) relative to the second representative value is equal to or more than a predetermined second threshold, determining that the gap information acquired for the bottom (Pb) is abnormal.