An image processing device (1, 1A) comprising: a main body for implementing at least one of a copy function, a scanner function, a facsimile function and a printer function; an engine unit including, at least, a plotter; an operation unit for operating the at least one of the copy function, the scanner function, the facsimile function and the printer function; an input receiver configured to receive an input of an authentication pattern that is a trajectory of contact coordinates indicating a contact position on the operation unit; an authentication application that is installed in the operation unit; and a concatenation image display unit configured to display an image connecting, out of a plurality of predetermined images that are displayed on the operation unit, a part of the predetermined images that includes a fist predetermined image and a second predetermined image of the predetermined images that are connected by the trajectory, wherein, prior to a display area of the first predetermined image overlapping the trajectory of the contact coordinates, a display area of the second predetermined image overlaps the trajectory of the contact coordinates, wherein, upon detecting that the authentication pattern input through the operation unit matches a registered authentication pattern, the authentication application is configured to allow a function of the engine unit to be used, wherein the authentication application is configured to cause only a single line image between two predetermined images of the plurality of predetermined images to be displayed, the two predetermined images being attempted to be connected by a user; and the authentication application is configured to cause all other line images to be deleted during displaying the single line image, characterized in that the authentication application is configured to display a registration screen for registering user information for inputting an input field for inputting a user ID, an input field for inputting a user name, and an input field for inputting user's mail address, the registration screen further displaying an authentication pattern input button, wherein upon detecting that the authentication pattern input button is operated, an authentication processing unit implemented by the authentication application determines that a request to input an authentication pattern is received, and the authentication processing unit causes the screen to transition to an input screen for inputting the authentication pattern, wherein the mail address to be input is used for transmitting information, to a user, from the authentication processing unit and the mail address is also used for transmitting data of a document scanned by the image processing device (1, 1A). a main body for implementing at least one of a copy function, a scanner function, a facsimile function and a printer function; an engine unit including, at least, a plotter; an operation unit for operating the at least one of the copy function, the scanner function, the facsimile function and the printer function; an input receiver configured to receive an input of an authentication pattern that is a trajectory of contact coordinates indicating a contact position on the operation unit; an authentication application that is installed in the operation unit; and a concatenation image display unit configured to display an image connecting, out of a plurality of predetermined images that are displayed on the operation unit, a part of the predetermined images that includes a fist predetermined image and a second predetermined image of the predetermined images that are connected by the trajectory, wherein, prior to a display area of the first predetermined image overlapping the trajectory of the contact coordinates, a display area of the second predetermined image overlaps the trajectory of the contact coordinates, wherein, upon detecting that the authentication pattern input through the operation unit matches a registered authentication pattern, the authentication application is configured to allow a function of the engine unit to be used, wherein the authentication application is configured to cause only a single line image between two predetermined images of the plurality of predetermined images to be displayed, the two predetermined images being attempted to be connected by a user; and the authentication application is configured to cause all other line images to be deleted during displaying the single line image, characterized in that the authentication application is configured to display a registration screen for registering user information for inputting an input field for inputting a user ID, an input field for inputting a user name, and an input field for inputting user's mail address, the registration screen further displaying an authentication pattern input button, wherein upon detecting that the authentication pattern input button is operated, an authentication processing unit implemented by the authentication application determines that a request to input an authentication pattern is received, and the authentication processing unit causes the screen to transition to an input screen for inputting the authentication pattern, wherein the mail address to be input is used for transmitting information, to a user, from the authentication processing unit and the mail address is also used for transmitting data of a document scanned by the image processing device (1, 1A).