A light quantity adjusting method is used for driving a light emitting device with two drive current levels. The drive current levels are controlled by the first set data and the second set data, respectively. First, a resolution ratio and light quantity characteristics are stored. The resolution ratio represents a ratio between a drive current setting resolution of the first set data and a drive current setting resolution of the second set data. The light quantity characteristics represents a relationship between the first set data and a light emission quantity of the light emitting device. A value in the first set data is obtained as a first set value through adjustment for a quantity of light. Next, a change amount, from the first set value in the first set data is calculated using the light quantity characteristics. The calculated change amount is converted into a value in the second set data as a second set value using the resolution ratio.