a thread cutting device of a sewing machine comprises : a movable knife having a thread pickup portion and being movable between a standby position and a thread pickup position ; a fixed knife being movable between a standby position and an operating position close to a needle locating point and cutting sewing threads in cooperation with the movable knife at the operating position ; a front stopper for holding the fixed knife at the standby position , wherein the front stopper is released from the fixed knife in the course of the movable knife making a forward movement ; and a rear stopper for holding the fixed knife at the operating position , wherein the rear stopper is released from the fixed knife in the course of the movable knife making a backward movement and after the sewing threads have been cut .