A base material processing apparatus comprising: a transport mechanism for transporting an elongated strip-shaped base material in a longitudinal direction thereof along a predetermined transport path; a processing part for processing the base material in a predetermined processing position lying on said transport path; a first detector for acquiring a variation with time in a first detection value indicative of the amount of widthwise misregistration of the base material in a first detection position lying on said transport path; a second detector for acquiring a variation with time in a second detection value indicative of the amount of widthwise misregistration of the base material in a second detection position lying on said transport path and downstream of said first detection position, characterised in that the base material processing apparatus further comprises: a detection position coefficient calculation part for determining a first coefficient obtained when each of the variation with time in said first detection value and the variation with time in said second detection value is applied to a predetermined model function; and a processing position coefficient calculation part for calculating a second coefficient of said model function in said processing position, based on said first coefficient and a positional relationship between said first detection position, said second detection position and said processing position. a transport mechanism for transporting an elongated strip-shaped base material in a longitudinal direction thereof along a predetermined transport path; a processing part for processing the base material in a predetermined processing position lying on said transport path; a first detector for acquiring a variation with time in a first detection value indicative of the amount of widthwise misregistration of the base material in a first detection position lying on said transport path; a second detector for acquiring a variation with time in a second detection value indicative of the amount of widthwise misregistration of the base material in a second detection position lying on said transport path and downstream of said first detection position, characterised in that the base material processing apparatus further comprises: a detection position coefficient calculation part for determining a first coefficient obtained when each of the variation with time in said first detection value and the variation with time in said second detection value is applied to a predetermined model function; and a processing position coefficient calculation part for calculating a second coefficient of said model function in said processing position, based on said first coefficient and a positional relationship between said first detection position, said second detection position and said processing position. a detection position coefficient calculation part for determining a first coefficient obtained when each of the variation with time in said first detection value and the variation with time in said second detection value is applied to a predetermined model function; and a processing position coefficient calculation part for calculating a second coefficient of said model function in said processing position, based on said first coefficient and a positional relationship between said first detection position, said second detection position and said processing position.