NOVELTY - An active substance comprises secondary particles constructed by aggregated primary particles of an active material comprising a niobium composite oxide and having compression fracture strength of 10 MPa or more, and a carbon material phase formed on portion(s) of surface of secondary particles.    USE - Active substance is used for nonaqueous electrolyte battery e.g. lithium-ion secondary battery used for battery pack used in automobiles (all claimed) e.g. hybrid vehicles and electric cars. Can also be used as power supply for mobile phone base station.    ADVANTAGE - The active substance has excellent conductivity, and provides battery having high current performance and capacity, and excellent cycle characteristics.    DETAILED DESCRIPTION - An active substance comprises secondary particles constructed by aggregated primary particles of an active material comprising a niobium composite oxide of formula: LixM(1-y)NbyNb2O(7+d), and having compression fracture strength of 10 MPa or more, and a carbon material phase formed on portion(s) of surface of secondary particles.    M=Ti or Zr;    x=0.1-6;    y=0-1;and    d=(-1)-1.    INDEPENDENT CLAIMS are included for the following:    (1) nonaqueous electrolyte battery (100), which comprises a positive electrode, a negative electrode comprising the active substance, and a nonaqueous electrolyte;    (2) battery pack, which comprises the nonaqueous electrolyte battery; and    (3) production of active substance, which involves providing a niobium composite oxide according to a solid phase reaction, grinding the niobium composite oxide to provide a ground product, forming the ground product into secondary particles, sintering, arranging a carbon precursor on a surface of each of the sintered secondary particles to form a composite comprising the secondary particles and the carbon precursor, and carbonizing the carbon precursor.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the flat-type nonaqueous electrolyte battery.    Coiled electrode group (1)    Baggy container (2)    Negative electrode terminal (6)    Positive electrode terminal (7)    Battery (100)