A tremolo unit mechanism for an electric guitar includes a bridge base vertically disposed to be pivotal about a support point on a body, balancing springs for causing a balance moment along a direction opposite to that of a moment due to a tension of strings to act on the bridge base, bridge main bodies, disposed on the bridge base, for holding one end of each of the strings, octave adjusting screws, threadably engaged with the bridge base, for moving the bridge base along a front-to-rear direction, a tremolo arm, mounted in the bridge base, for moving the bridge base vertically, and a control mechanism for controlling pivotal movement of the bridge base. The control mechanism includes an engaging member for engaging with the bridge base and a screw member for controlling pivotal movement/displacement of the engaging member.