securely inserted into the cartridge 1 as shown in fig8 c as the first arm 17a continues swinging clockwise the arms 17a and 18a become reversely doglegged after passing the straight state formed by the two arms 17a and 18a and stops swinging upon contact with the stopper 23a in this stage the force fs which presses the arm 18a to the stopper 23a can always be obtained without a rotational torque from the threading motor 21 which swings the first arm 17a therefore the threading mechanism can maintain a stable self locking state the threading motor 21 stops rotating when the sensor 15b detects a flag 16b thus if the sensor 15b continues to detect the flag 16b with the threading mechanism in a stable self locking state it is determined that the leader block 3 has been securely inserted into the cartridge 1 according to the present invention the threading arm which includes a linkage of two arms one of which includes an elastic member absorbs a shock caused when inserting a leader block inserts the leader block with a small rotational torque and also maintains itself in a stable self locked state at the leader block pickup position the sensor assures leader block insertion by detecting the threading arm in the self locked state accordingly leader block insertion can be stably and economically accomplished by a tape threading mechanism having the simple configuration of the present invention the foregoing is considered as illustrative only of the principles of the present invention further since numerous modifications and changes will readily occur.