PURPOSE: To realize not only optional change of an angle made between wing surfaces but also its easy control by forming a spreader expansible.

CONSTITUTION: When a spreader 4 is fixed in the predetermined length through tightening a band 43 by means of a screw 44 and fixing an end part member 45 on the periphery of an inner pipe 49, a cubical wing is formed. In case of changing an angle made between canvasses 21 and 22, inner and outer pipes 48, 49 are expanded or shrunk through being slided with each other after slackening of the band 43, and when the spreader comes to a predetermined length, it is fixed by means of the band 43. Expansion and shrinkage of the spreader, and a resultant change of the angles made between spurs 3 and between struts 5, are both performed by rotation of couplers 6, 7 round a keel 2.