A flame-retardant rigid polyurethane foam contains a flame retardant, the foam having a ratio of the maximum peak intensity ratio (P1) of the foam after moist heat treatment of the foam for one week at a temperature of 80° C. and a humidity of 85% to the maximum peak intensity ratio (P2) of the foam before this moist heat treatment of 85% or more (P1/P2x100). The P1 and P2 each refer to the ratio of the maximum peak intensity of 1390 to 1430 cm−1 to the maximum peak intensity of 1500 to 1520 cm−1 when the infrared absorption spectrum is measured at a position 5 to 10 mm from the surface of the foam, and the average intensity of 1900 to 2000 cm−1 is adjusted to zero.