The purpose of this study is to clarify the relationship among the viscosity of fluid diet and mechanism of the gastric myoelectrical activity with simultaneous measurements of electrogastrography (EGG) and heart rate variability (HRV). We studied for 8 healthy male volunteers, EGG and electrocardiogram (ECG) were measured simultaneously. And indexes of EGG and HRV for the periods of fasting and postprandial obtained by the analysis using Short-time Fourier transform (STFT) to assess gastric myoelectrical activity and autonomic nervous activity respectively on the intakes of three kinds of test meals which were different in viscosities. In relation to indexes of EGG, dominant power (DP) of CH3 and CH4 changed significantly (p < 0.05, 0.01) on the intake of low or high viscosity meal. In addition, dominant frequency deviation (DFD) of CH1 and CH2 changed significantly (p < 0.05, 0.01) only on the intake of low viscosity meal. In relation to indexes of HRV, LF/HF of only 15-20 min on postprandial period changed significantly (p < 0.05) on the intake of high viscosity meal. These results may express that distal stomach has a main role in gastric emptying of high viscosity meal similar to known mechanism of gastric emptying, whereas the intake of low viscosity meal lead to irregular activity on proximal stomach.