To provide a main spindle head easily adjusting the reduction ratio of a spindle head.

This main spindle head furnished with: the main spindle 10 to install a tool on it free to connect to and disconnect from it; and a speed reducer 30 to reduce rotating speed from a motor 20, is constituted so that: the main spindle 10 and an output shaft (third rotating shaft) 33 of the speed reducer 30 are connected to each other through couplings 44, 45 free to connect to and disconnect from each other; a main spindle storage part 11 to support the main spindle 10 free to rotate and a speed reducer storage part 34 to store the speed reducer 30 are formed as separate bodies; and the main spindle storage part and the speed reducer storage part 34 are connected to each other free to connect to and disconnect from each other.