A transmission apparatus includes a scanner. A first control is performed about reading of a plurality of pages of documents by using the scanner to generate image data. A second control is performed about conversion processing of the generated image data. A third control is performed about transmission of processed data on which the conversion processing is performed. The third control includes starting a session while a document is being read based on the first control, and transmitting, to an external apparatus via a network, the processed data on which conversion is already performed. Where an error occurs in the scanner in reading the documents, the third control further includes disconnecting the started session. Where the error is resolved, the third control further includes connecting the session again and transmitting, to the external apparatus via the network, processed data on which the conversion processing is performed.