To provide a key cylinder which can enhance the water-proof ability of a tumbler part and the burglar-proof ability.

An automobile key cylinder is composed of a hollow casing 5 having a large diameter aperture part 15 opened at the front end and formed in its lower part with a drain hole 21a, a small diameter aperture 19 formed in its upper and lower parts with inward tumbler grooves 5a, 5b, and fixed in a substantially horizontal direction, and a cylinder 7 having a cylinder head part 25 and the tumbler part 27 fitted in the casing 5. A weir part parting the tumbler groove 5b from the drain hole 21 is formed on the front end part of the small diameter aperture part 19, and an arcuate casing side seal surface 15a is formed at the front end of the weir part. The upper half part of the large diameter aperture part 15 is set to be deeper than the lower half part thereof so as to define a cicumferential stopper surface 17 at the laterally upper end of the seal surface 15a, and an arcuate seal surface 25a facing the seal surface 15a and having a length longer than that of the seal surface 15a, is formed in the rear part of the cylinder head part 25. Further, a stopper surface 25b adapted to make contact with the stopper surface 17 during rotation of the cylinder is formed in the upper part of the cylinder head part 25.