Linear motion mechanism sealants capable of operating at low torque when engaging in linear motion which are capable of carrying out sealing with respect to fine particulate several μm in diameter, and which are side seal units for sealing gaps at a shaft provided at a front end portion and a back end portion of a slider of a linear motion mechanism equipped with said shaft and said slider which carries out reciprocating linear motion on said shaft, the linear motion mechanism side seal units 4 being characterized in that fibrous surfaces of seal members comprising fibrous material are arranged so as to be directed toward said shaft in such fashion as to abut and conform to the cross-sectional shape of said shaft at the front end portion and the back end portion of the slider.