A tensioner device for a wrapping connection driving device in an internal combustion engine in which an endless transmission belt is extended between a driving wheel and a driven wheel supported on an engine body, and a tensioner is pressed against the transmission belt on a loosening side therof to apply a predetermined tension force to the transmission belt, wherein said tensioner device has a hydraulic automatic adjuster comprising a cylinder bore formed in a casing provided on said engine body, a plunger case fitted in said cylinder bore, a plunger slidably fitted in said plunger case, a high pressure oil chamber defined below the plunger, an extra feed oil chamber provided within said plunger to communicate with said high pressure oil chamber through a valve port bored in a bottom wall of said plunger, a check valve encased in said high pressure oil chamber to close said valve port when pressure in said high pressure oil chamber increases and to open the valve port when pressure in said oil chamber decreases, and a tension spring encased in said high pressure oil chamber to bias said plunger in an extending direction to apply a predetermined tension force to said transmission belt on the loosening side thereof through said tensioner, wherein said extra oil chamber is communicated via a feed oil port and an oil discharge port with a feed oil source and an oil reservoir, respectively, through respective closed oil paths, a seal member being provided to oil-tightly seal between an open end of said cylinder bore and said plunger case, and a flexible boot being provided to oil-tightly seal between an open end of said plunger case and an outer end of said plunger.