The invention provides a catalyst for thermal decomposition of an organic substance having the form of spherical granule containing a titanium/niobium composite oxide and having a particle diameter of 0.1 to 1.2 mm, a pore volume of 0.1 to 0.3 mL/g, a tap density of 1.05 to 1.4 g/mL, and a wear rate of 2% by weight or less, and further provides a method for pyrolysis of organic substances using the above catalyst.