In an inkjet printer and the like, by ejecting an ink (recording material) from a plurality of nozzles (recording elements) an image is formed on a recording sheet (recording medium). 
As such the printer, an image recording device exists that utilizes a long line head to cover the main scanning direction of the recording sheet.  With this type of image recording device, recording is performed in the main scanning direction with a state of fixing the line head, and by conveying the recording sheet in the perpendicular direction (sub-scanning direction) to the line head direction (main scanning direction), the image can be formed. 
Here, the log line head to cover the width of the recording sheet has weak points compared to the short head such as high manufacturing cost, low yield ratio at manufacturing, low reliability, further, need of total expensive line head replacement in case of breakage of a part of the recording elements, and high repairing cost is required. 
Against these problems, a method is proposed in Patent Document 1 (Examined Japanese Patent Application Publication No.  H04-38589) where a long line head is configured by arranging a plurality of short heads in the direction of nozzle array as shown in FIG.  10. 
Although the basic problem can be solved in such the configuration, a new problem is caused.  Namely the adjustment between each head is very difficult, and in case of insufficient adjustment, a line shaped false contour is generated at the nozzle boundary area between each short head. 
Recently in particular, since resolution of image print tends to increase, measures to cope with this tendency are imperative.  For example, in the case of printing with the resolution of 1440 dpi (dpi: dots/inch), the pitch of the recording elements becomes 17 μm. 
Against the new problems in the case of configuring a long head by combining short heads, various proposals are available.  For example they are: to thin out pixels in the main scanning direction, to thin out in the sub-scanning direction, to stir the boundary, to shift overlapping position by each color in case of color head, and the like. 
Further, in Patent Document 2 (Japanese Registration patent No.  3702711), proposed is a method where several recording elements of the short head are overlapped, and the elements are allocated regularly to each head according to the positions, or allocated to disperse irregularly by utilizing random numbers.