A contactless power feeding system comprising: a primary circuit configured to supply primary power to a power feeding coil; and a primary control circuit configured to control the primary circuit, the primary control circuit being configured to acquire a primary measured value as a measured value of the primary power, the primary control circuit being configured to acquire a secondary measured value and a secondary target value from a secondary control circuit, the secondary measured value being a measured value of secondary power received by a power receiving coil to be electromagnetically coupled to the power feeding coil, the secondary target value being a target value of the secondary power, the primary control circuit being configured to alternately and repeatedly perform: control processing of controlling the primary circuit to bring the primary measured value into conformity with a primary target value that is a target value of the primary power; and update processing (S 24) of updating the primary target value, wherein the update processing includes processing of calculating the primary target value based on transfer efficiency to be determined by the primary measured value and the secondary measured value and on the secondary target value, characterized by the primary control circuit being configured to perform, after the control processing and before performing the update processing (S 24), constant power control processing (S 23), wherein the constant power control processing (S 23) includes controlling the primary circuit to maintain the primary measured value at the primary target value for a predetermined period of time before the primary target value is updated during update processing (S 24); wherein the primary control circuit is further configured to acquire the secondary measured value from the secondary control circuit during a period in which the constant power control processing (S 23) is performed. a primary circuit configured to supply primary power to a power feeding coil; and a primary control circuit configured to control the primary circuit, the primary control circuit being configured to acquire a primary measured value as a measured value of the primary power, the primary control circuit being configured to acquire a secondary measured value and a secondary target value from a secondary control circuit, the secondary measured value being a measured value of secondary power received by a power receiving coil to be electromagnetically coupled to the power feeding coil, the secondary target value being a target value of the secondary power, the primary control circuit being configured to alternately and repeatedly perform: control processing of controlling the primary circuit to bring the primary measured value into conformity with a primary target value that is a target value of the primary power; and update processing (S 24) of updating the primary target value, wherein the update processing includes processing of calculating the primary target value based on transfer efficiency to be determined by the primary measured value and the secondary measured value and on the secondary target value, control processing of controlling the primary circuit to bring the primary measured value into conformity with a primary target value that is a target value of the primary power; and update processing (S 24) of updating the primary target value, wherein the update processing includes processing of calculating the primary target value based on transfer efficiency to be determined by the primary measured value and the secondary measured value and on the secondary target value, characterized by the primary control circuit being configured to perform, after the control processing and before performing the update processing (S 24), constant power control processing (S 23), wherein the constant power control processing (S 23) includes controlling the primary circuit to maintain the primary measured value at the primary target value for a predetermined period of time before the primary target value is updated during update processing (S 24); wherein the primary control circuit is further configured to acquire the secondary measured value from the secondary control circuit during a period in which the constant power control processing (S 23) is performed.