Abstract: A bolt tightening device having an inner socket and an outer socket arranged coaxially with each other and coupled to a motor by planetary gear mechanisms for tightening a reaction-bearing bolt with a nut thereon by engaging the nut in the outer socket and a tip at one end of the bolt in the inner socket. The inner and outer sockets are coupled respectively to two output shafts of the gear mechanism in the terminal stage which are rotatable in directions opposite to each other. The terminal gear mechanism has an axis displaced from the axes of the other gear mechanisms in parallel thereto, and has two input shafts coupled to two output shafts of the other gear mechanisms by way of first and second idle gears, respectively. This construction reduces the axial thickness of the socket side of the device, rendering the device easy to use.