An image processing apparatus which is capable of transmitting document data in a proper document format to an external system. An original input section inputs document data. A document data transmission section transmits the input document data to the system. A workflow execution section executes a workflow defined as a series of processes that can include a process for inputting the document data, and a process for transmitting the document data to the external system. A document format acquisition section acquires a document format of the document data required by the system, from the system. When the workflow execution section executes the workflow including the process for transmitting the document data to the external system, a document format check section judges whether or not the document data to be transmitted satisfies the acquired document format.