A light amount adjusting device comprising: a light amount adjusting unit configured to adjust an amount of illumination light emitted by a light source unit; an optical sensor configured to detect a brightness of the illumination light and a light intensity of each of all color components of red, green, and blue included in the illumination light; a gain adjusting unit configured to adjust gain values of each of the color components output from a light-modulation element which is configured to light-modulate each of the color components of red, green, and blue included in the illumination light and output a projection video, characterized in that, at a first time point, the light amount adjusting unit is configured to store the brightness of the illumination light as a first brightness for the projection video to have a desired brightness, and store a light intensity of each of the color components of the illumination light as a first light intensity, the gain adjusting unit is configured to set the projection video to have a desired color and set a gain value of at least one of the color components to be a maximum value, and store the set gain value as a first gain value, at a second time point after the first time point, the light amount adjusting unit is configured to adjust the amount of the illumination light so that the brightness of the illumination light emitted from the light source unit coincides with the first brightness, and calculate a ratio between a second light intensity and the first light intensity, the second light intensity being a light intensity of each of the color components of the illumination light emitted from the light source unit, the gain adjusting unit is configured to correct the gain values based on the ratio between the second light intensity and the first light intensity, and if the corrected gain values exceed the maximum value, reduce the gain values of the color components at the same ratio so that the largest gain value among the gain values of the color components becomes the maximum value and the light amount adjusting unit is configured to increase the amount of the illumination light. a light amount adjusting unit configured to adjust an amount of illumination light emitted by a light source unit; an optical sensor configured to detect a brightness of the illumination light and a light intensity of each of all color components of red, green, and blue included in the illumination light; a gain adjusting unit configured to adjust gain values of each of the color components output from a light-modulation element which is configured to light-modulate each of the color components of red, green, and blue included in the illumination light and output a projection video, characterized in that, at a first time point, the light amount adjusting unit is configured to store the brightness of the illumination light as a first brightness for the projection video to have a desired brightness, and store a light intensity of each of the color components of the illumination light as a first light intensity, the gain adjusting unit is configured to set the projection video to have a desired color and set a gain value of at least one of the color components to be a maximum value, and store the set gain value as a first gain value, at a second time point after the first time point, the light amount adjusting unit is configured to adjust the amount of the illumination light so that the brightness of the illumination light emitted from the light source unit coincides with the first brightness, and calculate a ratio between a second light intensity and the first light intensity, the second light intensity being a light intensity of each of the color components of the illumination light emitted from the light source unit, the gain adjusting unit is configured to correct the gain values based on the ratio between the second light intensity and the first light intensity, and if the corrected gain values exceed the maximum value, reduce the gain values of the color components at the same ratio so that the largest gain value among the gain values of the color components becomes the maximum value and the light amount adjusting unit is configured to increase the amount of the illumination light.