A handwriting input device for fax application connected to either the modem of a mobile phone or a built-in radio communication module having a function equivalent to a modem, whereby a user can send the content of a handwriting image to a remote fax machine by firstly writing a digital pen on a digital tablet, secondly transforming the recorded image into a fax format by a fax format transformation procedure, and eventually sending out the image by a mobile phone or a built-in radio communication module.