Electron density irregularity in the scale size of 4 to 100 m was observed in altitude between 90 and 120 km from rocket measurements over Japan. Analysis of the data makes clear that the spectral index of irregularities is 2 to 4 and that the irregularity amplitude is smaller than 5%. Strong irregularities exist at 92 to 95 km. The generation mechanism of these irregularities is explained by the theories of the cross field instability or the neutral turbulence.