A lubricating device used in a linear transmission mechanism is disclosed to include a housing having an accommodation chamber and an opening formed on the inside and periphery thereof in equal cross section, a pressure member axially movable in the accommodation chamber, a screw rod pivotally mounted in the housing and having a first transmission portion located on one end thereof and extended out of the pressure member, a screw nut threaded onto the screw rod and driven by the screw rod to stop against the pressure member outside, a transmission member defining therein a transmission slot and a second transmission portion and a third transmission portion at two opposite sides of the transmission slot for alternatively rotating the first transmission portion, and driving device for reciprocating the transmission member.