3. The electronic illustrated dictionary device according to claim 2 , further comprising: a link destination headword highlighter adding unit configured to execute a process of, when the in-illustration textual content display unit displays a expository content having a character string as a headword not highlighted with a highlighter by the illustration highlighter adding unit on the display unit, highlighting the headword of the displayed expository content with a highlighter according to a user operation; and a link originating illustration highlighter adding unit configured to execute a process of, when the link destination headword highlighter adding unit highlights the headword of the expository content with a highlighter, highlighting a character string in an illustration from which a link with the headword originates with a highlighter.
3. The electronic illustrated dictionary device according to claim 2 , further comprising: a link destination headword highlighter adding unit configured to execute a process of, when the in-illustration textual content display unit displays a expository content having a character string as a headword not highlighted with a highlighter by the illustration highlighter adding unit on the display unit, highlighting the headword of the displayed expository content with a highlighter according to a user operation; and a link originating illustration highlighter adding unit configured to execute a process of, when the link destination headword highlighter adding unit highlights the headword of the expository content with a highlighter, highlighting a character string in an illustration from which a link with the headword originates with a highlighter. 5. The electronic dictionary device according to claim 3 , wherein: the link destination headword highlighter adding unit executes a process of, when the in-illustration textual content display unit displays an expository content having a character string as a headword not highlighted with a highlighter by the illustration highlighter adding unit on the display unit, highlighting a headword of the displayed expository content with a highlighter of an arbitrary color according to a user operation, and the link originating illustration highlighter adding unit executes a process of, when the link destination headword highlighter adding unit highlights the headword of the expository content with a highlighter, highlighting a character string in an illustration from which a link with the headword originates with a highlighter of the same color.