Mobile robotics is one of the most studied scientific and technological fields, which is still in progress. Several research interests such as path planning, point stabilization, localization, obstacle avoidance and passable gap detection are commonly studied fields. Gap detection task affects the path planning characteristics of a mobile robot. Especially under presence of limited information about robot’s environment, passable gap detection is necessary for steering the mobile robot towards a goal autonomously. This paper concentrates on passable gap detection for unconstructed environments, which contain only positive obstacles. The method considers specific obstacle configurations such as presence of wall-type obstacle, maze type environments and random placed small sized obstacles. The method proposed in this study is based on reading distance of the obstacles in a certain range and detecting the borders of passable gaps. The detected gaps are re-organized depending on the priority assigned by the robot’s passage order of the gaps. The proposed scheme not only utilizes simple derivation of the measurement data but also extracts hidden gaps in the environment. The proposed scheme assumes the mobile robot is equipped with laser range sensor (LIDAR). A real LIDAR is modelled and adapted to the developed algorithm. The algorithm was developed in Matlab.