A dope containing a TAC and solvent is prepared in a dope producing apparatus. In a case where a new dope having components different from that of the dope described above is produced, a controlling device performs controlling such that an operation of each pump is stopped, each valve is set to a closing position, and a hopper stops supplying the TAC. Thereafter, a washing liquid is introduced into the respective devices of the dope producing apparatus. The dope remained in the respective devices are discharged as a waste liquid together with the washing liquid. The waste liquid is heated in dry gas until a liquid level of the waste liquid turns into a falling-rate drying state in a dry heating chamber provided in a solvent removing device. In a wet heating chamber, the waste liquid is heated in wet gas.