1. An image forming method in which a sample is illuminated with sinusoidally spatially modulated illuminating light, light from the sample illuminated by the illuminating light is focused as an image, and a sample image is formed by image calculation processing from the acquired image, wherein a greater number of images than the number of spatially modulated components contained in the light emitted from the sample as a result of the illuminating light on the sample are acquired, and the sample image is formed by applying the method of least squares, which is shown by the following equation and in which complex numbers are considered, ( ∑ j ⁢ ⁢ b 1 ⁢ ⁢ j ⁢ I kj ⁡ ( k ) ∑ j ⁢ ⁢ b 0 ⁢ ⁢ j ⁢ I kj ⁡ ( k ) ∑ j ⁢ ⁢ b + 1 ⁢ ⁢ j ⁢ I kj ⁡ ( k ) ) = ( ∑ j ⁢ ⁢ b - 1 ⁢ ⁢ j 2 ∑ j ⁢ ⁢ b - 1 ⁢ j ⁢ b 0 ⁢ j ∑ j ⁢ ⁢ b - 1 ⁢ j ⁢ b + 1 ⁢ j ∑ j ⁢ ⁢ b 0 ⁢ j ⁢ b - 1 ⁢ j ∑ j ⁢ ⁢ b 0 ⁢ j 2 ∑ j ⁢ ⁢ b 0 ⁢ j ⁢ b + 1 ⁢ j ∑ j ⁢ ⁢ b + 1 ⁢ j ⁢ b - 1 ⁢ j ∑ j ⁢ ⁢ b + 1 ⁢ j ⁢ b 0 ⁢ j ∑ j ⁢ ⁢ b + 1 ⁢ j 2 ) ⁢ ( O k ⁡ ( k - K ) ⁢ P k ⁡ ( k ) O k ⁡ ( k ) ⁢ P k ⁡ ( k ) O k ⁡ ( k + K ) ⁢ P k ⁡ ( k ) ) ( 1 ) where b lj =m l exp(iφ j ), m l is a modulated amplitude of the illuminating light, φ j is a modulated phase of the jth one of images, I kj (k) is a signal intensity of the jth one of images, and P k (k) is a transfer function of the optical system, with the amount by which the plurality of picked-up images are varied when these images are picked up for each of the spatial frequency components constituting the plurality of picked-up images taken as a parameter.