Abstract: In a drag-free joint assembly: a stationary body 1 is embedded flush in a connected object 10; a rotary shaft 7 is embedded flush in the body 1; a spring 4 is in a cavity 15 of the shaft 7; a locking shaft 2 is inserted in an upper end of the cavity 15 flush with the shaft 7; a lock pin 3 passes through an axially-elongated through-hole 17 of the shaft 7 and is inserted into a through-hole 16 of the shaft 2; a notch 8 is in an inner surface of the body 1 to permit an end of the pin 3 to move when the shaft 2 is rotated; the notch 8 has an axially-extending locking groove 9 in an end of its bottom surface; the groove 9 engages with the end of the pin 3; and a hole 11 is in the other object 12; whereby the pin 3 engages with the groove 9 when the shaft 7 is rotated to have the plate 5 crossed with the hole 11 at a right angle behind the hole.