A rotary type coupler for a shoulder strap assembly or a bag with an adjustable shoulder strap assembly comprising: a first coupling module for coupling a first upper strap to a first lower strap to be rotatable and having a first locking portion; and a second coupling module for coupling a second upper strap to a second lower strap to be rotatable and having a second locking portion and detachably locked to the first locking portion, wherein the first coupling module and the second coupling module each further include: an upper plate (210, 310) adapted to be connected to the first upper strap or the second upper strap; a lower plate (230, 330) adapted to be connected to the first lower strap or the second lower strap; and a connecting pin (250, 350) disposed to face the lower plate (230, 330) from the upper plate (210, 310) and coupling the upper plate (210, 310) to the lower plate (230, 330) to be rotatable, and the first locking portion has a plug coupled to the upper plate of the first coupling module, and the second locking portion has a receptacle coupled to the upper plate of the second coupling module, characterized in that the first coupling module further includes a first cover disposed over the plug to cover the plug, and in that the plug has a pair of hooks adapted to be locked by the receptacle, the pair of hooks being disposed at the free end of the plug at both sides of the connecting pin, and the first cover includes a circular plate having a pair of cut-off portions corresponding to the pair of hooks and exposing the pair of hooks. a first coupling module for coupling a first upper strap to a first lower strap to be rotatable and having a first locking portion; and a second coupling module for coupling a second upper strap to a second lower strap to be rotatable and having a second locking portion and detachably locked to the first locking portion, wherein the first coupling module and the second coupling module each further include: an upper plate (210, 310) adapted to be connected to the first upper strap or the second upper strap; a lower plate (230, 330) adapted to be connected to the first lower strap or the second lower strap; and a connecting pin (250, 350) disposed to face the lower plate (230, 330) from the upper plate (210, 310) and coupling the upper plate (210, 310) to the lower plate (230, 330) to be rotatable, and an upper plate (210, 310) adapted to be connected to the first upper strap or the second upper strap; a lower plate (230, 330) adapted to be connected to the first lower strap or the second lower strap; and a connecting pin (250, 350) disposed to face the lower plate (230, 330) from the upper plate (210, 310) and coupling the upper plate (210, 310) to the lower plate (230, 330) to be rotatable, and the first locking portion has a plug coupled to the upper plate of the first coupling module, and the second locking portion has a receptacle coupled to the upper plate of the second coupling module, characterized in that the first coupling module further includes a first cover disposed over the plug to cover the plug, and in that the plug has a pair of hooks adapted to be locked by the receptacle, the pair of hooks being disposed at the free end of the plug at both sides of the connecting pin, and the first cover includes a circular plate having a pair of cut-off portions corresponding to the pair of hooks and exposing the pair of hooks.