Attachment coupler

An attachment coupler that is attached to an end of an arm of a working machine, an attachment having two attachment pins in parallel being detachably attached to the attachment coupler, the attachment coupler comprising:

• a pair of locking plates, each one of the locking plates comprising
• a first U-shaped portion that engages with and supports one of the two attachment pins sideways;
• and
• a second U-shaped portion that has an opening that opens downwardly, the other of the two attachment pins being received by the opening upwardly,
• a rotary hook rotatably supported between two second U-shaped portions of the locking plates and that opens and closes the opening of the second U-shaped portions;
• the rotary hook including:
• an engagement securing member that engages with and secures the rotary hook when the rotary hook closes the opening of the second U-shaped portion and when the rotary hook opens the opening of the second U-shaped portion;
• and
• a bolt securing member that secures the rotary hook with a bolt passing through aligned holes of the rotary hook and the locking plates when the rotary hook closes the opening of the second U-shaped portion,
• the engagement securing member including:
• a lock level tool that is spring-biased to protrude toward the rotary hook;
• and
• locking depressions that are formed in the rotary hook, the lock level tool engaging with one of the locking depressions when the rotary hook closes the opening of the second U-shaped portion, and engaging with another one of the locking depressions when the rotary hook opens the opening of the second U-shaped portion.