On the basis of a large number of experiments conducted at many places in the country, it was clarified that when the temperature is low during the ripening period, Tachigaren application is very effective in increasing rate of ripening and promoting grain development of rice. It is, therefore, desirable to apply Tachigaren at the full heading stage when an undesirably low temperature during the ripening period is expected, or when the heading is delayed for one week or more due to a low temperature before heading. This chemical may also be appreciated in the regions where unripe or waste rice grain problems occur chronically.