The compound and its manufacturing method provided in a highly flame-retardant and good physical property when used as a flame-retardant finishing agent for fibers, obtained in an industrially advantageous productivity excellent method. The compound is a pentaerythritol diphosphonate represented by formula (1), (i) having a volume-based median diameter of 30 μm or less, (ii) a purity of 98% or more, and (iii) containing the specific halogenated compound at 200 ppm or less. (Ar1 and Ar2 are each independently a substituted/unsubstituted aryl group of C6-20; R3-R6 are each independently H, a substituted/unsubstituted aryl group of C6-20, or a saturated/unsaturated straight/branched alkyl group of C1-20) No selection figure.