A contactless power transmission circuit for transmitting power in a contactless state to a power receiving device placed on a placing portion, comprising: a resonant circuit including a power transmission coil for transmitting power to the power receiving device and a resonant capacitor which resonates with the power transmission coil; a switching element for resonating the resonant circuit by being repeatedly turned on and off; and a driving circuit for detecting whether or not the power receiving device is placed on the placing portion based on a voltage of the power transmission coil, and driving the switching element such that an ON-period of the switching element is shorter when the power receiving device is detected not to be placed than when the power receiving device is placed, wherein the driving circuit includes: a self-oscillating circuit for repeatedly turning the switching element on and off based on the voltage of the power transmission coil to cause the self-oscillation of the resonant circuit; and an intermittent driving circuit for intermittently driving the self-oscillating circuit when the power receiving device is detected not to be placed on the placing portion, wherein the self-oscillating circuit causes the self-oscillation of the resonant circuit for a period during which the intermittent driving circuit keeps the switching element on, and does not cause the self-oscillation of the resonant circuit for a period during which the intermittent driving circuit keeps the switching element off. a resonant circuit including a power transmission coil for transmitting power to the power receiving device and a resonant capacitor which resonates with the power transmission coil; a switching element for resonating the resonant circuit by being repeatedly turned on and off; and a driving circuit for detecting whether or not the power receiving device is placed on the placing portion based on a voltage of the power transmission coil, and driving the switching element such that an ON-period of the switching element is shorter when the power receiving device is detected not to be placed than when the power receiving device is placed, wherein the driving circuit includes: a self-oscillating circuit for repeatedly turning the switching element on and off based on the voltage of the power transmission coil to cause the self-oscillation of the resonant circuit; and an intermittent driving circuit for intermittently driving the self-oscillating circuit when the power receiving device is detected not to be placed on the placing portion, wherein the self-oscillating circuit causes the self-oscillation of the resonant circuit for a period during which the intermittent driving circuit keeps the switching element on, and does not cause the self-oscillation of the resonant circuit for a period during which the intermittent driving circuit keeps the switching element off. a self-oscillating circuit for repeatedly turning the switching element on and off based on the voltage of the power transmission coil to cause the self-oscillation of the resonant circuit; and an intermittent driving circuit for intermittently driving the self-oscillating circuit when the power receiving device is detected not to be placed on the placing portion, wherein the self-oscillating circuit causes the self-oscillation of the resonant circuit for a period during which the intermittent driving circuit keeps the switching element on, and does not cause the self-oscillation of the resonant circuit for a period during which the intermittent driving circuit keeps the switching element off.