A solid laser amplification device comprising: a laser medium part which has a solid medium, configured to be entered by a laser light from an entrance part on a side surface of the solid medium, to let the laser light travel through the laser medium part and to emit the laser light from an exit part on a side opposite to the entrance part to the outside, and an amplification layer which is respectively provided on a lower bottom surface and an upper bottom surface of the solid medium, is configured to receive the laser light in the solid medium, and to amplify and reflect the laser light toward the exit part; a microchannel type cooling part which is configured to cool the amplification layer; and a thermally conductive part being a plate-shaped member extending in horizontal directions x and z and having a thickness extending in a vertical direction y, which is provided in contact with the amplification layer and the microchannel type cooling part between the amplification layer and the microchannel type cooling part and is configured to transfer heat of the amplification layer to the microchannel type cooling part, wherein the microchannel type cooling part has a microchannel part which is a plate-shaped member having a plurality of flow paths opened therein and extending along a direction parallel to a surface of the microchannel part and allows a cooling solvent to flow therethrough, and wherein a) the thermal conductivity of the thermally conductive part along the direction x and z is higher than thermal conductivity along the direction y, or b) the thermal conductivity of the thermally conductive part along the direction x and z is lower than thermal conductivity along the direction y. a laser medium part which has a solid medium, configured to be entered by a laser light from an entrance part on a side surface of the solid medium, to let the laser light travel through the laser medium part and to emit the laser light from an exit part on a side opposite to the entrance part to the outside, and an amplification layer which is respectively provided on a lower bottom surface and an upper bottom surface of the solid medium, is configured to receive the laser light in the solid medium, and to amplify and reflect the laser light toward the exit part; a microchannel type cooling part which is configured to cool the amplification layer; and a thermally conductive part being a plate-shaped member extending in horizontal directions x and z and having a thickness extending in a vertical direction y, which is provided in contact with the amplification layer and the microchannel type cooling part between the amplification layer and the microchannel type cooling part and is configured to transfer heat of the amplification layer to the microchannel type cooling part, wherein the microchannel type cooling part has a microchannel part which is a plate-shaped member having a plurality of flow paths opened therein and extending along a direction parallel to a surface of the microchannel part and allows a cooling solvent to flow therethrough, and wherein a) the thermal conductivity of the thermally conductive part along the direction x and z is higher than thermal conductivity along the direction y, or b) the thermal conductivity of the thermally conductive part along the direction x and z is lower than thermal conductivity along the direction y. a) the thermal conductivity of the thermally conductive part along the direction x and z is higher than thermal conductivity along the direction y, or b) the thermal conductivity of the thermally conductive part along the direction x and z is lower than thermal conductivity along the direction y.