To simplify the structure of a latch mechanism.

The latch mechanism 31 for detaining a slider 3 to a lower plate 4 is composed of only the two parts; a latch 32 and a latch spring 33. When a user raises a safety cap 14 upward, the latch 32 is pressed down and turned by a pressing part 24a and removes the latch pawl 32a from a detaining hole 3b of the slider 3 and therefore the extension movement of the slider 3 is made free. When the user detaches the finger from the safety cap 14 to make the cap free, the latch 32 is turned in a backward direction by the force of the latch spring 33 and again the latch spring 33 is detained into the detaining hole 3b.