A weft knitting machine with a movable sinker device capable of preventing yarn from being worn caused by repeatedly striking the yarn with a sinker energized by a spring to push down knitting fabric, wherein a stroke to which a yarn locking part (2a) at the tip of a sinker plate (2) is moved to a tooth port (6) by the energization of the spring (4) is increased at a first stage shown in (a) and decreased at a second stage shown in (b), the stroke is limited in (b) by locking a holding locking part (12a) of a holding member (12) to the holding locking part (2d) of the sinker plate (2), and a stage is switched to a second stage at a knitting portion where the yarn locking part (2a) repeatedly strikes the yarn to prevent the yarn from being thinned and fluffed.