To provide a rotation transmission device dispensing with backlash adjusting work done by a skilled worker and having simple constitution and a monitoring device using the rotation transmission device.

This rotation transmission device includes; a worm base 2 rotatably holding a worm 1; a worm wheel base 7 rotatably holding a worm wheel 5; guide mechanisms 8, 9 for moving the worm base in the normal direction intersecting perpendicularly to the axis of the worm wheel; and a coiled spring 10 disposed between the worm base and the worm wheel base and having spring force in the normal direction of the worm wheel, wherein the center distance between the worm and the worm wheel is kept within a predetermined range by following the force in the normal direction acting on the tooth flanks of the worm and the worm wheel.