To determine any chipping of a fillet roller with good accuracy with simple control.

The maximum pressure change ΔPmaxj and the integrated pressure change value ΔPj are calculated by using the sequentially calculated pressure change ΔPi per predetermined time (Steps S102-S114), the permissible ranges ΔPjmav±3σsj and ΔPmaxjmav±3σmj are set by using nearest 20 sets of integrated pressure change value ΔPj-19 to ΔPj and the maximum pressure change value ΔPmaxj-19 to ΔPmaxj including the maximum pressure change ΔPmaxj and the integrated pressure change ΔPj (Steps S114-S118), and it is determined whether or not any chipping is caused in a fillet roller R depending on whether or not the maximum pressure change value ΔPmaxj and the integrated pressure change value ΔPj are within the range of the permissible value range ΔPmaxjmav±3σmj and ΔPjmav±3σsj, respectively (Step S120).