An object of the present invention is to provide a continuous mixing plant that, for example, simply continuously feeds the necessary materials while metering the amount thereof and drops these materials by the gravitational force so that the mixed material may be manufactured continuously for a short period of time suitably. As a means for attaining this object, a continuous mixing plant comprising a continuous metering and feeding means for keeping on feeding at least two kinds of materials to be mixed with each other while continuously metering the materials, the number of the continuous metering and feeding means being the number corresponding to the materials, and at least one mixing box unit for mixing the materials fed continuously from the continuous metering and feeding means, is characterized in that the mixing box unit is provided with: a plurality of modified passages each of which has an inlet at one end and an outlet at the other end and a cross-sectional shape of which is continuously changed from the inlet toward the outlet and which extend in an axial direction; and a merging and dividing means provided between the inlet and the outlet of each of the modified passages for merging and dividing each material passing through each of the modified passages, and each material is continuously cast from the inlet portion and passed toward the outlet portion through each of the modified passages by the gravitational force to be mixed. In the case where the mixed material is concrete, in particular, each material is continuously measured with high precision and fed to a mixer to thereby make it possible to continuously produce the high quality concrete for a short period of time.