PURPOSE: To prevent a user from being kept waiting without executing copying in a correction mode by stopping succeeding correction in the case that a measured correction quantity is converged to a specified quantity or under.

CONSTITUTION: A correction term Δt which is a point is obtained by the difference of the detected temperature (ts) detected at the contact position and the non-contact position of a thermistor with the roller. When Δt is within the range of a fixed value, it is compared with the previous Δt. In the case that the difference is equal to or under the fixed value, for example, 1°C, Δt is not updated by judging that the inner part of a fixing unit becomes a thermal equilibrium state, and the measurement of Δt is not executed next. In the case that the difference exceeds the fixed value, Δt is updated to Δt which is newly measured. Thus, the user is prevented from being kept waiting without executing the copying in the correction mode.