The invention provides a water-ingress-preventing mechanism for a lotion pump. The mechanism is mainly used for a lotion pump mounted on a container with a liquid therein and is able to effectively prevent foreign liquids from entering the container. According to one aspect of the invention, in the locking tube portion (13) of the press head (10) is formed at least one longitudinal slot (15) which extends upwards from the lower end edge of the locking tube portion over at least part of the length of the locking tube portion. The slot is disposed such that the upper end of the slot is at a position above the top surface (25) of the cylinder cover (20) when the press head is pressed down to and at the lower stop position of its press stroke with respect to the cylinder cover. According to another aspect of the invention, the external thread (14) on the locking tube portion extends helically upwards from the lower end edge of the locking tube portion, whereas the internal thread (26) in the central through-hole (21) in the cylinder cover extends helically downwards from the upper end edge of the central through-hole. The features in the both of the two aspects effectively prevent foreign liquids from being sucked into the container during operation of the lotion pump.