A solid-state laser device (1A-1G), comprising: a laser medium (2A-2E) including an upper surface (13A - 13D), a bottom surface (14A - 14D), an input surface (15A - 15D) and an output surface (15A - 15D) whereby on the light input and output surfaces (15A-15D) at least one of incidence and emission of light is to occur; an inner container in which the laser medium (2A-2E) is accommodated and which includes a first inner window and a second inner window which are configured to transmit incident light incident on the light input and output surfaces (15A-15D) and emission light emitted from the light input and output surfaces (15A-15D); an outer container in which the inner container is accommodated, and which includes at least a first outer window and a second outer window which are provided at a part that faces the first inner window and the second inner window, respectively, transmitting the incident light and the emission light, the inner container and the outer container being vacuum-insulated from each other; a cooling medium supply unit configured to supply a cooling medium inside the inner container so that the cooling medium comes in contact with at least a part of a surface other than the light input and output surfaces (15A-15D) in the laser medium (2A-2E); and a cover section (6, 6D) that partitions light-passing areas, through which the incident light and the emission light between the light input and output surfaces (15A-15D) and the first inner window and the second inner window pass, from cooling medium supply areas to which the cooling medium is supplied from the cooling medium supply unit, the cooling supply areas being an upper area provided above the upper surface (13A - 13D) bounded by the upper surface (13A - 13D) and the inner surface of the inner container, and a lower area provided below the lower surface (13A - 13D) bounded by the lower surface (14A - 14D) and the inner surface of the inner container, and wherein the laser medium (2A-2E) is accommodated in the inner container so that the cooling medium supply areas and the light-passing areas are under the same pressure. a laser medium (2A-2E) including an upper surface (13A - 13D), a bottom surface (14A - 14D), an input surface (15A - 15D) and an output surface (15A - 15D) whereby on the light input and output surfaces (15A-15D) at least one of incidence and emission of light is to occur; an inner container in which the laser medium (2A-2E) is accommodated and which includes a first inner window and a second inner window which are configured to transmit incident light incident on the light input and output surfaces (15A-15D) and emission light emitted from the light input and output surfaces (15A-15D); an outer container in which the inner container is accommodated, and which includes at least a first outer window and a second outer window which are provided at a part that faces the first inner window and the second inner window, respectively, transmitting the incident light and the emission light, the inner container and the outer container being vacuum-insulated from each other; a cooling medium supply unit configured to supply a cooling medium inside the inner container so that the cooling medium comes in contact with at least a part of a surface other than the light input and output surfaces (15A-15D) in the laser medium (2A-2E); and a cover section (6, 6D) that partitions light-passing areas, through which the incident light and the emission light between the light input and output surfaces (15A-15D) and the first inner window and the second inner window pass, from cooling medium supply areas to which the cooling medium is supplied from the cooling medium supply unit, the cooling supply areas being an upper area provided above the upper surface (13A - 13D) bounded by the upper surface (13A - 13D) and the inner surface of the inner container, and a lower area provided below the lower surface (13A - 13D) bounded by the lower surface (14A - 14D) and the inner surface of the inner container, and wherein the laser medium (2A-2E) is accommodated in the inner container so that the cooling medium supply areas and the light-passing areas are under the same pressure.