A method for welding synthetic resin members includes the steps of detecting a temperature of a high frequency electrode or an ultrasonic horn (or an anvil) by a temperature detector, cooling the high frequency electrode or the ultrasonic horn (or the anvil) by cooling means when the temperature detected by the temperature detector during a high frequency or ultrasonic vibration exceeds a preset temperature range, thereby reducing the temperature rapidly down to the preset temperature range, and stopping an operation of the cooling means when the temperature of the high frequency electrode or the ultrasonic horn (or the anvil) is reduced to the preset temperature range.