A connecting device connects a first attachment and a second attachment constituting a working machine with each other. The connecting device includes a first connected member, a second connected member, and a connecting member (connecting pin 15). The first connected member extends outward from a first side surface of the first attachment, the first side surface extending in a longitudinal direction of the first attachment. The second connected member extends outward from a second side surface of the second attachment, the second side surface extending in a longitudinal direction of the second attachment, the second connected member facing the first connected member when the first attachment and the second attachment come into a connection state. The connecting member connects the first connected member and the second connected member with each other.