A wet-type image forming apparatus and method is provided to sense and maintain a toner concentration level of a developer, and includes a counter for counting the number of dots of input print data, a memory for storing an average coverage of a print medium which corresponds to a predetermined dot number of the print data and storing a lookup table for consumption quantities of carrier and toner which correspond to the average coverage, and a control unit for calculating a toner concentration of a developer that remains in a developer supplying unit by recognizing consumption quantities of the carrier and the toner that are stored in the memory based on the counted dot number.