A casing structure of a vehicular power transmitting device, includes: a casing; a breather plug; and a stationary removal preventive member, wherein the breather plug includes an attaching portion and a body portion, the body portion of the breather plug including an abutting section which abuts on the removal preventive member when the attaching portion of the breather plug is moved relative to the communication hole in a direction toward the outside atmosphere. The removal preventive member is positioned relative to the communication hole such that the attaching portion of the breather plug is located to keep the breather plug in communication with the communication hole when the abutting section of the body portion comes into abutting contact with the removal preventive member.