To provide a halogen-free flame-retardant rigid polyurethane foam capable of satisfying necessary and regulated requirements for flame retardance, processability, low density of smoke, and low toxicity of the smoke, and foamed by water.

This halogen-free flame-retardant rigid polyurethane foam contains an oxyalkylated alkylphosphonic acid in combination with an ammonium polyphosphate as a flame retardant. The alkylphosphonic acid is, for example, exemplified by the formula [R1 is a methyl, ethyl or propyl group; R2 and R3 are same or different and each a methyl, ethyl or propyl group; and x is a number of 1.2-1.9].