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Effect of viscosity of an epoxy near or over its gel point on foaming structures was investigated. Rheological properties of the sample with a weight ratio 100/1 of epoxy/curing agent were measured by a simple shear experiment with sinusoidally varying shear. The magnitude of complex viscosity η* as a function of time increases rapidly after a definite time period from the time when the temperature is set to 90 °C and then increases slowly after η* reached about 1×106 Pa⋅s. The sample is found to gelate in the range of time t = 15.0∼15.6 min by the oscillatory measurement. The samples with a weight ratio 100/1/0.5 of epoxy/curing agent/blowing agent were foamed by putting them for 5 min in a furnace of 230 °C after various precuring conditions at 90 °C. There are roughly two sizes of bubbles, large bubbles (>100 μm) and small ones (≅30 μm), when the sample is precured for relatively short time before foaming. On the other hand, foams precured for a long time before foaming, large sizes of bubbles disappea...