To provide a back-raising device for a bed which is advantageous for smooth operations at the initial stage of starting the back-raising operation of a back-raising driving arm.

The back-raising mechanism for the bed has a back-raising mechanism 20A having a back-raising driven arm 300 for turning in a back-raising direction and a back-lowering direction, following the back-raising driving arm. The back-raising driven arm 300 has a lumbar support part 350 having a contactor 360 capable of sliding along a bottom 103 for the lumbar support along with the back-raising operation of the back-raising mechanism. The bottom 103 for the lumbar support has a rail 60. The rail 60 approaches or comes into contact with the contactor 360 of the lumbar support part 350 at the initial stage of starting the back-raising operation of a back-raising driving arm while opposing the contactor and is extended toward the bottom 103 for the lumbar support.