conversion dial 201 will longitudinally move the main change over arm 104 through the stitch conversion cam 202 conversion lever 203 and link means 106 in fig3 a first movement of the main change over arm 104 shown in solid line as indicated by an arrow a when performing 34 roll hem stitching 34 while a second motion or displacement of the main change over arm shown in phantom line as indicated by an arrow mark b occurs when performing 34 double chain stitching 34 the dial 201 cam 202 lever 203 and link means 105 are designed so that when the dial 201 is rotated the change over operation between the lower looper oscillating device 20 and lower looper back and forth oscillating device 21 together with control for the upper looper oscillating device 23 are performed corresponding to various stitch formation such as over edge stitch and double chain stitch or the like the dial 201 is preferably provided with indications representing the respective stitch formations for the convenience of selection thereof as will be appreciated from the foregoing conversion of various stitch formations may be readily made by simply rotating the dial 201 operation of the overlock sewing machine according to the invention will be explained below it should be noted that in general the lower looper 2 is only oscillated by the lower looper oscillating device when performing 34 over edge stitching 34 in this case the lower looper 2 is prevented from back and forth movement while the upper looper 3 is actuated by the upper looper oscillating device 23 accordingly 34 over edge stitching 34 may be performed by means of oscillational.