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New phenomena in producing sheet-shaped plasma under electron cyclotron resonance (ECR) condition have been found and investigated by experiment. Sheet-shaped Ar plasma with density of ∼1018 m-3 was successfully obtained at ∼10-3 torr by using 2.45-GHz microwave with ECR condition. The plasma was generated uniformly and stably in the cross section of 15.5 cm width×2.0 cm thickness with the use of waveguide of the rectangular cross section under the quartz window in vacuum. The parameter region of stable sheet-shaped plasma production was made clear by experiments clarifying criteria in parameter map. It was found that the magnetic field strength corresponding to the position and the shape of ECR region was the most effective parameter affecting microwave propagation to control the shape of plasma. However, the uniformity of the sheet-shaped plasma width was poor over the distance of microwave wavelength λ under 1 kW of input power. Therefore, it was concluded that the microwave was absorbed quickly near ECR region and hardly propagates farther than ∼λ to the direction of perpendicular to the magnetic field. The double sheet-shaped plasma was produced at some region and two peaks were relatively shifted to the direction of E×B. Also, the condition of sheet-shaped plasma production was different for helium gas. By the electron density measurement and semi-one-dimensional analysis, the differences of ionization cross section and diffusion coefficient perpendicular to the magnetic fields were important factors for electron density and the shape of sheet plasma. The qualitative interpretation of sheet-shaped plasma production is discussed.