To effectively prevent picking.

There is provided a cylinder lock for a building, which includes a holder 1 and a cylinder main body 2. The holder 1 is mounted in a door main body and has first and second engaging grooves in its internal peripheral surface. The cylinder main body 2 is rotatably arranged in the holder 1, and provided with a plurality of tumblers 12 and a side bar 13. The tumblers 12 are energized in a direction orthogonal to an insertion direction of a key in the cylinder main body 2 to protrude from the same, and then engage with the first engaging groove of the holder 1. The side bar 13 is energized toward the tumblers 12, and before insertion of the key, the side bar 13 abuts on side edges of the tumblers and protrudes from the cylinder main body 2, followed by engaging with the second engaging groove 9 of the holder 1. Then, upon insertion of the key, the tumblers 12 are kept to sink into the cylinder main body 2, and the key engages with a notch 23, to thereby sink into the cylinder main body 2.