The invention relates to a device for forming contraction joints in concrete works. The invention comprises a plurality of assemblies which are made from a concrete-separating material and which are mounted to rigid linear elements in an alternate manner on one side and then the other of the surface crack line on the upper face of the concrete work surface, leaving spaces therebetween for cracking. The inventive device enables the adjacent slabs formed by the contraction joint to be fixed, thereby improving the resistance performance thereof. The invention is suitable for in situ concrete works, such as roads, channels, esplanades, sewers, tunnels, railways, dykes, etc.