In a radiating structure for a main spindle having a tapered bore formed at a distal end of the main spindle, into which a tapered portion of a tool holder is mounted, a key fixing member provided with a drive key is fixed to the distal end of the main spindle, and the drive key is fit into a key groove formed in the tool holder. This configuration prevents a phase shift in rotation direction of the tool holder arising from rotation of the main spindle. A radiating member which exchanges heat with an ambient atmosphere is provided to the key fixing member provided with the drive key.