When a cultivation condition value (X) changed by using a prescribed value (PV) in a changing step (S3) is equal to or above a preset upper limit value (UL), the cultivation condition value (X) is replaced with the upper limit value (UL). When the cultivation condition value (X) changed by using the prescribed value (PV) in the changing step (S3) is equal to or below a preset lower limit value (LL), the cultivation condition value (X) is replaced with the lower limit value (LL).