A power transmitting device comprising: a power transmitting unit configured to contactlessly transmit alternating-current power to be transmitted to a power receiving unit of a power receiving device; and a controller configured to adjust a power transmission frequency representing a frequency of the power to be transmitted, characterized in that the controller being configured to (i) use information received from the power receiving device to determine whether the power receiving device has a function of adjusting a resonance frequency of the power receiving unit, and (ii) use a result of the determination to determine whether to perform the control applied to adjust the power transmission frequency, wherein the controller is configured to: when the power receiving device does not have the function of adjusting the resonance frequency, adjust the frequency of the power to be transmitted; and when the power receiving device has the function of adjusting the resonance frequency, (i) compare a controllability of the resonance frequency with a controllability of the power transmission frequency, and (ii) if the controllability of the power transmission frequency is higher than the controllability of the resonance frequency, adjust the power transmission frequency. a power transmitting unit configured to contactlessly transmit alternating-current power to be transmitted to a power receiving unit of a power receiving device; and a controller configured to adjust a power transmission frequency representing a frequency of the power to be transmitted, characterized in that the controller being configured to (i) use information received from the power receiving device to determine whether the power receiving device has a function of adjusting a resonance frequency of the power receiving unit, and (ii) use a result of the determination to determine whether to perform the control applied to adjust the power transmission frequency, wherein the controller is configured to: when the power receiving device does not have the function of adjusting the resonance frequency, adjust the frequency of the power to be transmitted; and when the power receiving device has the function of adjusting the resonance frequency, (i) compare a controllability of the resonance frequency with a controllability of the power transmission frequency, and (ii) if the controllability of the power transmission frequency is higher than the controllability of the resonance frequency, adjust the power transmission frequency. the controller being configured to (i) use information received from the power receiving device to determine whether the power receiving device has a function of adjusting a resonance frequency of the power receiving unit, and (ii) use a result of the determination to determine whether to perform the control applied to adjust the power transmission frequency, wherein the controller is configured to: when the power receiving device does not have the function of adjusting the resonance frequency, adjust the frequency of the power to be transmitted; and when the power receiving device has the function of adjusting the resonance frequency, (i) compare a controllability of the resonance frequency with a controllability of the power transmission frequency, and (ii) if the controllability of the power transmission frequency is higher than the controllability of the resonance frequency, adjust the power transmission frequency. when the power receiving device does not have the function of adjusting the resonance frequency, adjust the frequency of the power to be transmitted; and when the power receiving device has the function of adjusting the resonance frequency, (i) compare a controllability of the resonance frequency with a controllability of the power transmission frequency, and (ii) if the controllability of the power transmission frequency is higher than the controllability of the resonance frequency, adjust the power transmission frequency.