<p>A method of constructing an underwater tunnel allowing the tunnel to be efficiently constructed by using through- caissons having both end parts opened in extension direction, comprising the steps of arranging the plurality of through- caissons (1) having both end parts opened in extension direction at a water bottom while joining to the adjacent through-caissons (1), cutting out both end parts of a caisson group (10) having the plurality of through-caissons joined to each other from external water (22), and draining internal water (21) accumulated in the caisson group to bring the inside of the caisson group into an air-filled state.</p>
