© 2018 The Ceramic Society of Japan. All rights reserved. Suitable support materials for microorganisms are useful for bioreactors to achieve high efficiencies. Spherical porous hydroxyapatite (HA) granules about 5 mm in diameter composed of rod-like particles were prepared through hydrothermal treatment. The granules with and without pores tens of micrometers in size were prepared. These granules were immersed in a suspension containing microorganisms obtained from the soil, and the adhesion of microorganisms on the granules was examined. The adhesion of microorganisms on the surfaces of the HA granules was observed. The total metabolic activity of the microorganisms adhering onto the granule with pores tens of micrometers in size was higher than that adhering onto the granule without such pores. These results suggested that the HA granules had an affinity for microorganisms, and that granules with pores tens of micrometers in size are expected to be support materials for microorganisms.