A power transmitter apparatus for transmitting power to a power receiver apparatus comprising a power receiving coil, in a contactless manner, wherein the power transmitter apparatus comprises: a power transmitting coil, an auxiliary coil electromagnetically coupled to the power transmitting coil, a power supply circuit (11, 12) that generates transmitting power having a voltage and a frequency and supplies the transmitting power to the power transmitting coil, and a control apparatus characterized in that the control apparatus comprises: a first detector (13, 14) that detects a value of a current or voltage generated by the auxiliary coil; a second detector (13, 14) that detects a value of a current flowing through the power transmitting coil; a coupling coefficient estimator that estimates a first coupling coefficient between the power transmitting coil and the power receiving coil, based on the value of the current or voltage generated by the auxiliary coil, and estimates a second coupling coefficient between the power transmitting coil and the power receiving coil, based on the value of the current flowing through the power transmitting coil; and a control circuit that controls the power supply circuit (11, 12) to transmit power to the power receiver apparatus when a difference between the first and second coupling coefficients is equal to or less than a predetermined threshold, and stop power transmission to the power receiver apparatus when the difference between the first and second coupling coefficients is greater than the threshold. wherein the power transmitter apparatus comprises: a power transmitting coil, an auxiliary coil electromagnetically coupled to the power transmitting coil, a power supply circuit (11, 12) that generates transmitting power having a voltage and a frequency and supplies the transmitting power to the power transmitting coil, and a control apparatus characterized in that the control apparatus comprises: a first detector (13, 14) that detects a value of a current or voltage generated by the auxiliary coil; a second detector (13, 14) that detects a value of a current flowing through the power transmitting coil; a coupling coefficient estimator that estimates a first coupling coefficient between the power transmitting coil and the power receiving coil, based on the value of the current or voltage generated by the auxiliary coil, and estimates a second coupling coefficient between the power transmitting coil and the power receiving coil, based on the value of the current flowing through the power transmitting coil; and a control circuit that controls the power supply circuit (11, 12) to transmit power to the power receiver apparatus when a difference between the first and second coupling coefficients is equal to or less than a predetermined threshold, and stop power transmission to the power receiver apparatus when the difference between the first and second coupling coefficients is greater than the threshold. a first detector (13, 14) that detects a value of a current or voltage generated by the auxiliary coil; a second detector (13, 14) that detects a value of a current flowing through the power transmitting coil; a coupling coefficient estimator that estimates a first coupling coefficient between the power transmitting coil and the power receiving coil, based on the value of the current or voltage generated by the auxiliary coil, and estimates a second coupling coefficient between the power transmitting coil and the power receiving coil, based on the value of the current flowing through the power transmitting coil; and a control circuit that controls the power supply circuit (11, 12) to transmit power to the power receiver apparatus when a difference between the first and second coupling coefficients is equal to or less than a predetermined threshold, and stop power transmission to the power receiver apparatus when the difference between the first and second coupling coefficients is greater than the threshold.