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We present an overview of the preliminary design of the Telescope Structure System (STR) of Thirty Meter Telescope (TMT). NAOJ was given responsibility for the TMT STR in early 2012 and engaged Mitsubishi Electric Corporation (l 1 v1ELCO) to take over the preliminary design work. MELCO performed a comprehensive preliminary design study in 2012 and 2013 and the design successfully passed its Preliminary Design Review (PDR) in November 2013 and April 2014. Design optimizations were pursued to better meet the design requirements and improvements were made in t he designs of many of the telescope subsystems as follows:1. 6-legged Top End configuration to support secondary mirror (M2) in order to reduce deformation of the Top End and to keep the same 4% blockage of the full aperture as the previous STR design.2. "Double Lower Tube" of the elevation (EL) structure to reduce t he required stroke of the primary mirror (1tll) actuators to compensate the primary mirror cell (Ml Cell) deformation caused during the EL angle change in accordance with the requirements.3. Ml Segment Handling System (SHS) to be able to make removing and installing 10 Mirror Segment Assemblies per day safely and with ease over l:vll area where access of pers01mel is eA.i;remely difficult. This requires semi-automatic sequence operation and a robotic Segment Lifting Fh .. 'ture (SLF) designed based on the Compliance Control System, developed for controlling industrial robots, with a mechanism to enable precise control within the six degrees of freedom of posit ion control.4. C0 2 snow cleaning system to clean Ml every few weeks t hat is similar to t he mechanical system that has been used at Subaru Telescope.5. Seismic isolation and restraint systems with respect to safety; the maximtun acceleration allowed for Ml , M2, tertiary mirror (M3), LGSF, and science instruments in 1,000 year return period eaTthquakes are defined in the requirements.The Seismic requirements apply to any EL angle, regardless of t he operational st atus of Hydro Static Bearing (HSB) system and stow lock pins. In order to find a practical solut ion, design optimization study for seismic risk mitigation was carried out eA.i;ensively, including the performing of dynamic response analyses of t he STR system under the t ime dependent acceleration profile of seven major earthquakes. The work is now moving to the final design phase from April 2014 for two years.