In an image data transmission system, even when encoded serial digital data transmitted from an endoscope to a video processor 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 transmits error announcement information to a communication reception section in the endoscope based on a decoding error signal from a decoding error detection section, and a control code insertion section that has received the information instructs an encoding section to insert a control code for synchronization at a timing of occurrence of an error.