The stability of concrete-block work for the protection of the pier against the scour at flood is discussed by the way of the multivariate analysis.By using the data on the safety blocks and the flow-out blocks at flood-time, the discriminant analysis is executed.In conseqence, the discriminant function for the stability with the six variables is induced. Their variables are river width-depth ratio, river depth, friction velocity, river channel radius, block weight and construction width of block.An relation between scour depth around pier and construction width of block, a balanced weight and a balanced construction width are also discussed.Lastly, the judgement method about the stability on the concrete-block work is proposed with regard to the weight of anti-scour block and its construction width.