A controller obtains a load value within a bucket when a work implement has a basic attitude, as a reference value, and a load value, within the bucket when the work implement has an attitude different from the basic attitude, as a comparison value. The controller generates correction data according to an attitude of the work implement beforehand, based on the reference value and the comparison value. The controller senses a current attitude of the work implement, and calculates a correction amount of a load value within the bucket when the work implement has the current attitude, based on the correction data. The controller corrects a load value within the bucket when the work implement has the current attitude using the correction amount, and obtains a corrected load value.