To provide a retaining wall block which is reduced in constructing costs and material costs, improved in advantageousness of transportation, storage, etc. by miniaturing the structure as a unit body, available as a retrofitted block to an existing side ditch block, to thereby brings about simplification of work, recycling of a building material, and improvement in the general applicability thereof.

In manufacturing of the retaining wall block 1 which is to be installed in a gap between a road R and a slope S, the block is integrally formed of an abutting wall portion 2, a retaining wall portion 3, and a step engaging portion 4. The abutting wall portion abuts on a side surface As of the side ditch block A. The retaining wall portion is erected from the abutting wall portion 2 and has a predetermined height. The step engaging portion is formed between the retaining wall portion 3 and the abutting wall portion 2 so that it engages with an upper edge Au of the side ditch block A.