The detection of dioxins in municipal waste incineration fly ash in 1983 brought the problem of dioxins to the attention of Japanese society, but at the same time, the capacity of final disposal sites was becoming increasingly restricted. In 2002 and 2003, Musashino City conducted mixed incineration tests of plastic and other burnable refuse, and based on the measured dioxin emissions, it started to collect the plastic as burnable refuse for incineration from October 2003. Here, the change in dioxins concentrations since the start of mixed incineration is reported, together with data for the period from 1996 until the introduction of mixed incineration. Following the replacement of electric precipitators with bag-filters in 1999-2000, though the concentration of exhaust dioxins at the dust collector exit was drastically decreased, that at the stack remained higher than that at the dust collector exit. But it gradually decreased to 0.011 ng-TEQ/m(3)N (oxygen concentration 12% basis, average for three incinerators) in 2009, which is close to that at the dust collector exit and almost two orders of magnitude lower than the 0.97 ng-TEQ/m(3)N in 2000, when plastic was collected as incombustible refuse. These findings suggest that dioxins had earlier accumulated in high concentration in a stagnant zone in the flue gas duct from the dust collector exit to the stack and were gradually released after the start of mixed incineration.