To provide a preparation process for a fire retardant foam which has a high heat insulating property, a high compression strength and a low environment loading.

α,β-Unsaturated carboxylic acids, an α,β-unsaturated carboxylic acid (co)polymer, metal (hydrogen)carbonate and, if required, a polymerization initiator are continuously mixed. This mixed liquid is discharged into the double conveyor and, while restrained in the double conveyor, expanded, crosslinked with metal salt and, if required, polymerized to obtain a fire retardant board foam.