An automatic developer for developing a light-sensitive material such as a photographic material. The developer includes: a processor tank for accommodating a processing solution so that the processing solution processes the light-sensitive material; a throughput detector for detecting a throughput of the processor on the light-sensitive material, and for generating detection signals when the throughput becomes a predetermined value; a processing agent supplier for supplying a solid processing agent to the processor tank; a water supplier for supplying water to the processor tank; a calculator for calculating an amount of evaporation water of the processing solution; a timer for generating timer signals when a predetermined time period is passed from a reference time; a controller for controlling the processing agent supplier to supply the solid processing agent to the processor tank according to the detection signals, for controlling the water supplier to supply the water to the processor tank according to the detection signals in which an amount of the water for each time is not more than a predetermined supply amount, and for controlling the water supplier to supply the water corresponding the amount of evaporation water in accordance with the timer signals.