A vehicular air conditioning apparatus may include a case which forms a passage for conditioned air that flows toward a vehicle interior; a partition plate which divides a space inside the case; a sliding door which moves to change a flow of the conditioned air inside the case; a rack gear provided on the sliding door; and a pinion gear which moves the sliding door by rotating while being engaged with the rack gear. The partition plate may have a guide which slidably supports an end portion of the sliding door. The case may have a seal member, and the sliding door and the seal member may be disposed to slidably oppose each other. The guide may have a cut-out portion in which the seal member is disposed, and an auxiliary guide may be provided in the cut-out portion.