Laser flash method have been widely used for practical measuring method of thermal diffusivity. And it can be used for measurement of non-conductive materials as well as conductive materials and also for measuring thermal multi-properties. We have analyzed effects of error occurring during measurement of thermal properties in order to enhance measuring accuracy. Also we have studied delay time between measurement starting time with synchronizing signal and laser oscillating time, because it is important that measuring a time to rise temperature of specimen from room temperature to a half of measuring temperature at measuring a thermal diffusivity by laser flash method. We could reduce non-uniform heating error from non-uniform energy distribution by developing 3D uniformizer to eliminate non-uniform heating error. We have measured thermal diffusivities of POCO AXM-5Q1 and Glassy-Carbon which are standard specimen of NIST(USA) and candidate standard specimen of NRLM(japan) respectively for laser flash method. Maximum error fell within 2% for POCO AXM-5Ql gaphite and 2% for Glassy-Carbon. Those results showed error decreasing methods were effective.