In an image data transmission system, even when encoded serial digital data transmitted from an endoscope 2 to a video processor 3 cannot be decoded due to a shift in a delimiter position as a result of a lack of the serial digital data or the like, a communication transmission section 37 transmits error announcement information to a communication reception section 27 in the endoscope 2 based on a decoding error signal from a decoding error detection section 36, and a control code insertion section 25 that has received the information instructs an encoding section 23 to insert a control code for synchronization at a timing of occurrence of an error.
