PURPOSE: To find an accurate lens movement quantity in a short time from a detected defocusing quantity and improve the focusing precision and response of the automatic focusing device by correcting correction coefficients sequentially with next correction coefficients and an image plane deviation quantity and calculating a conversion coefficient.

CONSTITUTION: A storage means 4 is stored with plural correction coefficients for correcting the conversion coefficient according to the image plane deviation quantity, and a conversion coefficient calculating means 5 corrects the correction coefficients sequentially with next correction coefficients and the image plane deviation quantity and corrects a stored specific conversion coefficient with the correction coefficients and the image plane deviation quantity to calculate the conversion coefficient. This conversion coefficient is used to calculate the movement quantity of a focusing lens 2 from the image plane deviation quantity and the focusing lens 2 is driven and controlled, so the accurate movement quantity can be calculated from the defocusing quantity in a short time to improve the focusing precision and response.