Hi Ryo,
How much different are the dates in R/3 compared to APO? There are a lot of reasons the production orders dates could change in R/3. It is a sticky problem and depends upon your business scenario.
Try transaction OPU3 in R/3. Check that for your order type that scheduling is turned off for detailed scheduling and that dcheduling control doesn't have scheduling checked on. The transaction is by plant and order type.
Also, the settings in APO transaction /SAPAPO/CP3 could be useful.
It may help.
Best Regards,
Mike