There is provided a subminiature imaging optical system suitably applicable to a subminiature optical device such as a mobile phone camera. The subminiature imaging optical system including: a first lens having positive refractive power and both convex surfaces; a second lens having negative refractive power and a concave image-side surface; a third lens having negative refractive power and a meniscus shape with a convex image-side surface; and a fourth lens having negative refractive power.