A side sliding door apparatus for an electric railcar includes a side sliding door suspended from a horizontal door rail with a C-shape cross-section. The side sliding door is capable of moving freely via a door roller, and a gap is formed between the door roller and a ceiling of the door rail. A guide roller with a smaller diameter than that of the door roller is provided adjacent to the door roller. A gap between the guide roller and the ceiling of the door rail is set to be smaller than that between the door roller and the ceiling of the door rail. Preferably, a gap adjusting mechanism is provided for adjusting the gap between the guide roller and the ceiling of the door rail.