To manufacture aggregate usable as a material for civil work by making good use of the already developed technique to melt and solidify waste plastics.

A lump 1 of around 180-200°C obtained by melting waste plastics is made into a melted board 5 by passing through rolling rolls. The obtained board 5 is made into a stick-like body 9 by feeding between a pair of compressing/cutting rollers 8 for machine direction molding, which are provided with plural adjacent hollows 7 of a trapezoidal shape on the surfaces of the rollers in the longitudinal direction so that the surfaces of the rollers are brought into contact with each other. The body 9 is cut into crystalline block aggregate 11 by feeding between a pair of compressing/cutting rollers 10 for crosswise direction molding, which are provided with plural adjacent hollows 7 of a trapezoidal shape on the circumferential surfaces so that the surfaces of the rollers are brought into contact with each other. The aggregate 11 discharged from the rollers 10 is immediately cooled with water.