A pig pen structure has a walking passage and a plurality of pig houses for housing an animal installed along the walking passage. Each pig house has an evacuation room, a lodging room having front, back, and two side walls, and an evacuation room gate installed between the evacuation room and the lodging room. The evacuation room is accessible to the animal for excretion when the evacuation room gate is opened. The evacuation room gate when closed becomes a part of the back wall. A urine tube is installed on the bottom of the evacuation room, so that the excreted urine in the evacuation room flows down to the urine tube to remove the excreted urine outside the pig house. The plurality of the pig houses is arranged so that the front walls of the pig houses adjoin the walking passage. A continuous passage way is formed by the evacuation rooms along the back wall of the lodging room when the evacuation room gates of the pig houses are in closed position.