Hydraulic circuit, loader, and working machine

A hydraulic circuit comprising:

• an attachment cylinder to expand and contract to operate an attachment connected to a loader;
• a lock cylinder to expand and contract to be switchable between a fixing state in which the attachment is fixed to the loader and a releasing state in which the fixing of the attachment to the loader is released;
• an attachment oil passage to expand and contract the attachment cylinder by feeding hydraulic oil to the attachment cylinder;
• a lock oil passage to expand and contract the lock cylinder by feeding the hydraulic oil to the lock cylinder;
• and
• a connection oil passage that connects a first lock oil passage of the lock oil passage, through which the hydraulic oil to be fed to the lock cylinder flows in order to bring the lock cylinder into the fixing state, and the attachment oil passage.