In case of marine accident, the rapid search for missing object such as an underwater wreckage is one of the most important tasks. For this task, an autonomous underwater vehicle (AUV) that equips forward-looking imaging sonar can be utilized. The search task is accomplished by the following strategy. In the first phase, the AUV scans the target underwater area in a fast speed, and detects some suspected objects. In the second phase, the AUV approaches to the object and investigates it to determine whether the suspected object is the target object to be found. This paper provides experimental results of underwater object search in a fast speed for realizing the first phase of the strategy.