Main spindle device of machine tool

A main spindle device of a machine tool, comprising:

• a main spindle supported so as to be rotatable relative to a housing and having a first end to which a tool is attached;
• a rod accommodated in the main spindle so as to be coaxial therewith and capable of reciprocating in an axial direction with respect to the main spindle, the rod being capable of moving to an unclamp position where the tool is released in a direction toward the first end, and when the main spindle is holding the tool, being capable of moving to a clamp position where the tool is clamped in a direction toward a second end of the spindle, which is an opposite direction to the direction toward the first end;
• a collet that is attached to a first end of the rod, that holds the tool when the rod moves in the direction toward the second end with respect to the main spindle, and that releases the tool when the rod moves in the direction toward the first end with respect to the main spindle;
• a biasing unit that biases the rod in the direction toward the second end with respect to the main spindle and that biases the rod to the clamp position when the collet is holding the tool;
• and
• a biasing force detector that detects a biasing force of the biasing unit which biases the rod in the direction toward the second end after the rod is moved in the direction toward the first end with respect to the main spindle to the unclamp position by force larger than the biasing force of the biasing unit.