To provide an underwater observing device capable of easily and surely realizing an aritightness for an inner unit to be arranged inside a vessel while containing a cable for observation.

The underwater observing device 111 is composed of a column vessel 122 in which the inner unit 115 is disposed to observe. The column vessel 122 is hermetically sealed by a first cover 113 and a second cover 114, and furthermore, sealed by joint chamber covers 131C, 132C arranged inside joint rings 116, 117, the portion of which is fitted at the outside of the first and second covers 113, 114. A first and second submarine cables 141, 142 are separated to use for the inner unit and other than it in an inner space of the joint chamber 131. Cables 1412, 1422 being not used reach another inner space of the joint chamber 132 through a circuitous path of a penetration 151, set together with cables 1411, 1421 being used in the inner unit 115 and externally drawn out as a third submarine cable 143.