In S1, an image reading device obtains an image region length Ld of an original placed on an original mounting portion, a set moving velocity Vm of an image reading unit, a required deceleration distance Ls, a reference velocity Vr, and an absolute length Z. In S5, the image reading device sets a flag to 0 if Vm≦Vr (S2: NO) or Z≧(Ld+Ls) (S3: YES) and reads the entire image region length while the image reading unit is moving at the set moving velocity Vm. The image reading device sets the flag to 1 if Vm>Vr (S2: YES) and Z