An improved pump is provided that is capable of achieving excellent back suction function by using a depression head, without compromising the ease of depression operations of the head due to the additional function. The pump includes a depression head and an annular piston. The depression head is urged upward and is relatively displaceable with respect to the stem. When depressed, a lever member of the depression head operates to open a discharge opening. An annular piston opens a discharge valve, in an uppermost displacement position and closes the discharge valve in a lowermost displacement position. Back suction is caused due to a negative pressure state occurring in the stem until the discharge valve seat is closed by the annular piston during upward displacement of the stem.