In a robot servo mechanism, the servo mechanism is installed within a L-shaped housing box to make dimension thereof smaller with smaller dead space thereof. In the servo mechanism, each of first and second housing box 41 and 42 of a robot servo unit has a small motor, a reduction unit and a potentiometer. Since, each of the two housing boxes 41 and 42 has L-shaped configuration, each of corresponding connection spaces 41a and 42a surrounded by one dotted chain line can be formed. First and second output shafts 41b and 42b are installed with protrusion into the connection spaces 41a and 42a, respectively. A cross-shaped coupling body 50 having a cross-type through hole formed therein is fitted around the output shaft 41b along a line indicated by an arrow and then fixed thereat by using a screw. Then, the second output shaft 42b of the second housing box 42 is inserted into the cross-shaped coupling body previously fixed at the first housing box 41. As a result, the two housing boxes 41 and 42 are integrated as one body through the use of the cross-shaped coupling body 50, thereby constructing a joint servo of a robot.