An optical transmission device includes a splitter configured to have at least a first port, a second port, and a third port that output branched input light, branching ratios of the first port and the second port being variable, and a controller configured to reduce an optical level of output light from the first port to be monitored and increase an optical level of output light from the second port according to the reduced optical level of output light from the first port by controlling the branching ratios.