The present invention discloses an image data transmission apparatus. The lock confirmation circuit receives an original signal having an original pulse width from the image data reception apparatus to generate an output signal. The image data transmission circuit determines that the image data reception apparatus locks a transmission frequency when an output pulse width is larger than a pulse threshold value to perform a synchronous image data transmission. The lock confirmation circuit sets the output pulse width to be a lengthened pulse width when a difference between the original pulse width and the pulse threshold value is smaller than a predetermined value and the original pulse width is not smaller than the pulse threshold value. The lock confirmation circuit sets the output pulse width to be a shortened pulse width when the difference is smaller than the predetermined value and the original pulse width is smaller than the pulse threshold value.