A slim type ballpoint pen includes, a refill, a knock part, a spring and a spring stopper disposed in a shaft tube, a front hole or front slit and a rear slit 12 continuously provided in the shaft tube, and an insertion hole into which a button is inserted in the knock part, so that writing state is maintained by moving a rear end of the knock part or a projection on the knock part forward and projecting a middle body of the button upward at a rear end of the front hole or front slit, the button including an upper body part having the smallest diameter, a middle body part having a middle diameter and a lower body part having the largest diameter, and a not-writing state is maintained by projecting the upper body part upward at a rear end of the rear slit.