An illumination device, comprising: a sheet-shaped, flexible illumination panel including on its surface a light emitting face, said light emitting face being configured to emit light when supplied with electrical power; a main body including a mounting face whereon said illumination panel is to be mounted; and an installation cover being to be installed on said main body and to sandwich the illumination panel with said main body, said installation cover, in a state of being installed on said main body, including a pressing region pressing said light emitting face of said illumination panel, and said pressing region of said installation cover being configured to press said light emitting face of said illumination panel, thereby causing said illumination panel to be pressed against said mounting face of said main body, such that a shape of the light emitting face is defined and retained by a shape of the pressing region of the installation cover and a shape of the mounting face of the main body, characterized in that said light emitting face of said illumination panel is made of organic EL elements, and two supporting units are disposed side by side at one end of the main body and another end of the main body facing each other, respectively, an L-shaped retaining member is rotatably installed at at least two supporting units at one end of the main body, a biasing unit is built inside the supporting unit, and said biasing unit biases said retaining member to rotate toward said mounting face, thereby causing said pressing region of said installation cover to press said light emitting face of said illumination panel. a sheet-shaped, flexible illumination panel including on its surface a light emitting face, said light emitting face being configured to emit light when supplied with electrical power; a main body including a mounting face whereon said illumination panel is to be mounted; and an installation cover being to be installed on said main body and to sandwich the illumination panel with said main body, said installation cover, in a state of being installed on said main body, including a pressing region pressing said light emitting face of said illumination panel, and said pressing region of said installation cover being configured to press said light emitting face of said illumination panel, thereby causing said illumination panel to be pressed against said mounting face of said main body, such that a shape of the light emitting face is defined and retained by a shape of the pressing region of the installation cover and a shape of the mounting face of the main body, characterized in that said light emitting face of said illumination panel is made of organic EL elements, and two supporting units are disposed side by side at one end of the main body and another end of the main body facing each other, respectively, an L-shaped retaining member is rotatably installed at at least two supporting units at one end of the main body, a biasing unit is built inside the supporting unit, and said biasing unit biases said retaining member to rotate toward said mounting face, thereby causing said pressing region of said installation cover to press said light emitting face of said illumination panel.