The wasting of cutting fluid mists and contamination of a machining environment with cutting fluid mists are prevented by feeding optimum amounts of cutting fluid mists in a variety of machining operations. A main shaft device for machine tools, comprising a tool holder (8) fixed to a main shaft (2), the tool holder (8) being provided with a holder main body (8a), a chuck (18) and a cutting tool stopper member (20), it being arranged that cutting fluid mists inwardly fed into the main shaft (2) pass through a mist passageway formed in the cutting tool stopper member (20) and in a cutting tool (17), whereby they are spouted from the front end surface of the cutting tool (17), wherein the cutting tool stopper member (20) is composed of a main body member (22), and a front member (23) exchangeably fixed to the main body member (22) through a mounting/dismounting structure, and, in this case, the front member (23) serves as an adjusting nozzle member for controlling the flow rate of cutting fluid mists flowing out of the main body member (22) into the cutting tool (17).