Abstract: [Object] To provide a main-shaft and holder combination, a holder, and a machine tool which prevent a hydraulic fluid from leaking when the holder is detached from the main shaft, in a configuration for feeding the hydraulic fluid from the main shaft to the holder. [Solution Means] A main shaft fluid passage A30 is provided at a center of a spindle 25 and a main shaft check valve A11 is attached, and a holder fluid passage A32 is provided at a center of a holder 22 and a holder check valve A17 is attached. At a main shaft cylinder 23 position away from the spindle 25, a main shaft fluid passage B11 and a holder fluid passage B48 are provided, and a main shaft check valve B39 and a holder check valve B40 are attached. When the holder 25 is attached to the spindle 25, the main shaft check valve A11 (B39) and the holder check valve A17 (B40) are connected.

Abstract: Provided are: a double grasping mechanism including a double grasped part and a double gripping mechanism; a spindle, a holder holding mechanism, a main-spindle, and a holder holding device each including a double gripping mechanism; and a tool holder; a chuck holder; and a clamp holder. The double grasped part is formed by an outer pull stud and an inner pull stud. The double gripping mechanism is formed by: an outer gripping mechanism including an outer large-diameter part, an outer small-diameter part, an outer draw bar, and outer balls; and an inner gripping mechanism including an inner large-diameter part, an inner small-diameter part, an inner draw bar, and inner balls. The tool holder includes the outer pull stud. The chuck holder includes the outer pull stud and the inner pull stud. The clamp holder includes the outer pull stud and the inner pull stud.