Additive for synthetic resins and synthetic resin compositions

A synthetic resin composition which contains particles which are surface-coated with petaloid porous hydroxyapatite having a chemical formula Ca.sub.5 (PO.sub.4 ).sub.3 (OH), the petaloid porous hydroxyapatite being contained in the particles in a ratio of not less than 5% by weight of the particles, and the particles satisfying the following formulas (a)-(d):

• 0.1.ltoreq.dx1.ltoreq.20(.mu.m) (a)
• 1.ltoreq..alpha..ltoreq.2, where .alpha.=d50/dx1 (b)
• 0.ltoreq..beta..ltoreq.1.7, where .beta.=(d90-d10)/d50, and(c)
• 40/dx1.ltoreq.Sw1.ltoreq.400 (d)
• wherein,
• dx1:
• Average particle diameter (.mu.m) measured by a photograph of an electron microscope;
• .alpha.:
• Dispersion coefficient;
• d50:
• 50% average particle diameter (.mu.m) measured by a particle size distribution tester using a microtrack FRA laser;
• .beta.:
• Sharpness;
• a particle size distribution value;
• the smaller the value becomes, the sharper the distribution becomes;
• d90:
• 90% particle diameter of the total particles passed through a sieve measured by a particle size distribution tester using a microtrack FRA laser;
• d10:
• 10% particle diameter of the total particles passed through a sieve measured by a particle size distribution tester using a microtrack FRA laser;
• Sw1:
• BET specific surface m.sup.2 /g by a nitrogen adsorbing method.