To provide a method for dissolving an iridescent linear joining part observed under sunlight or an indoor fluorescent lamp, in a molding formed of an aromatic polycarbonate resin.

The method is a method for treating an injected molding formed of an aromatic polycarbonate resin, the method involving sandwiching the injected molding between two polarizing sheets and irradiating it with transmissive light to observe the iridescent linear joining part. Based on the birefringences in the transmission direction measured at measuring intervals of 5 mm in the range of 300 mm centered on the intersection of the iridescent linear joining part and a virtual reference line along the virtual reference line set to be orthogonal to the iridescent linear joining part, if the main stress at each measurement point obtained according to the Brewster's law is defined as S(i) (i is 1, 2, 3, ..., 61), and the absolute value of the gradient of the main stress [|(S(i)-S(i+1))|/5] obtained by dividing the difference between the main stress and the adjacent main stress S(i+1) by the measuring interval (5 mm) is defined as G(i), the value of G(i)+G(i+1)+G(i+2) is reduced to 0.4 by the execution of the treatment method.