A composite structure was recently patented. The structure includes an elongate core material of a sintered diamond comprising 80% by volume or more diamond particles of a mean particle size not larger than 3.5 μm and an iron group metal binding the diamond particles. A shell layer covers the circumference of the core material and comprises a sintered alloy of at least one kind of hard particles selected from among carbide, nitride and carbonitride of at least one metal element selected from the group of 4a, 5a and 6a group metals of the Periodic Table. The core material also comprises diamond particles of a mean particle size not larger than 5 μm. An iron group metal binds the hard particles and diamond particles. Content of the diamond particles included in the shell layer is from 5% to 45% by volume.