A color image forming device, comprising: a sheet carrying body configured to carry a sheet thereon in a predetermined direction; an image forming means comprising a process unit with development cartridges corresponding to different colors and configured to form an image on the sheet carried on the sheet carrying body, with an image forming property that determines image quality depending on a deviation of an image forming position of the image forming means; a pattern forming means configured to form a pattern in a region on a circumferential surface of the sheet carrying body, characterized by the sheet carrying body being divided in the circumferential direction into at least first and second pattern forming regions (A,C), the first region being shorter than a circumferential length of the sheet carrying body, and the pattern forming means being configured to form the pattern in the first and second regions (A,C) of the sheet carrying body and the pattern being provided with a plurality of marks formed in groups with kinds of marks for each of the colors of the development cartridges; a detection value determining means configured to determine a first detection value (Xc) representing an average positional deviation amount for each color based upon the positional deviation of marks of the pattern relative to the position of a mark of a predefined color through detecting the pattern formed in the first region on the circumferential surface of the sheet carrying body by the pattern forming means; a storage means (42, 43) configured to store thereon the first detection value (Xc) determined by the detection value determining means, a correction value determining means configured to determine a correction value for correcting the image forming position of the image forming means with the first detection value (Xc) stored on the storage means (42, 43) and a second detection value (Xa) that is a past detection value stored on the storage means at an earlier time than the first detection value (Xc), the second detection value (Xa) representing an average positional deviation amount for each color based upon the positional deviation of marks of the pattern relative to the position of a mark of a predefined color through detecting a pattern formed on the second region of the sheet carrying body, wherein the correction value is an average value between the second detection value (Xa) and the first detection value (Xc) and a control means configured to control the image forming means to form the image with an image forming property corrected based upon the correction value determined by the correction value determining means. a sheet carrying body configured to carry a sheet thereon in a predetermined direction; an image forming means comprising a process unit with development cartridges corresponding to different colors and configured to form an image on the sheet carried on the sheet carrying body, with an image forming property that determines image quality depending on a deviation of an image forming position of the image forming means; a pattern forming means configured to form a pattern in a region on a circumferential surface of the sheet carrying body, characterized by the sheet carrying body being divided in the circumferential direction into at least first and second pattern forming regions (A,C), the first region being shorter than a circumferential length of the sheet carrying body, and the pattern forming means being configured to form the pattern in the first and second regions (A,C) of the sheet carrying body and the pattern being provided with a plurality of marks formed in groups with kinds of marks for each of the colors of the development cartridges; a detection value determining means configured to determine a first detection value (Xc) representing an average positional deviation amount for each color based upon the positional deviation of marks of the pattern relative to the position of a mark of a predefined color through detecting the pattern formed in the first region on the circumferential surface of the sheet carrying body by the pattern forming means; a storage means (42, 43) configured to store thereon the first detection value (Xc) determined by the detection value determining means, a correction value determining means configured to determine a correction value for correcting the image forming position of the image forming means with the first detection value (Xc) stored on the storage means (42, 43) and a second detection value (Xa) that is a past detection value stored on the storage means at an earlier time than the first detection value (Xc), the second detection value (Xa) representing an average positional deviation amount for each color based upon the positional deviation of marks of the pattern relative to the position of a mark of a predefined color through detecting a pattern formed on the second region of the sheet carrying body, wherein the correction value is an average value between the second detection value (Xa) and the first detection value (Xc) and a control means configured to control the image forming means to form the image with an image forming property corrected based upon the correction value determined by the correction value determining means.