The present invention provides a tandem-type image forming method using a trickle developing system, wherein the average circularity of a toner contained in at least a supplementary developer used for appropriately supplying a developer to a developing device is in the range of 0.940 to 0.980. In the toner, the ratio of the number of particles having an average circularity of 0.970 or greater, in a particle diameter range of a toner circle-equivalent diameter.times.3/5 or less, is 5% or less, and the ratio of the number of particles having an average circularity of 0.950 or less, in a particle diameter range of a toner circle-equivalent diameter.times.7/5 or greater, is 10% or less.