1. A method of detecting exposure conditions of a charged particle exposure system which includes an electron gun that emits charged electron beams along an optical axis, a stage, a stencil mask positioned between the electron gun and the stage and having a surface which traverses the optical axis, an irradiation optical system positioned between the electron gun and the stencil mask, an incident mask deflector positioned between the electron gun and the stencil mask and which deflects electron beams from the optical axis towards the surface of the stencil mask, the electron beams passing through the stencil mask and being directed away from the optical axis after passing through the stencil mask, a reduction optical system positioned between the stencil mask and the stage, and an irradiation deflector positioned between the stencil mask and the stage and which deflects electron beams passing through the stencil mask towards the optical axis, the stencil mask having two mark patterns in a positional relationship, the method comprising the steps of: (a) operating the incident mask deflector to apply charged electron beams emitted by the electron gun to each of the mark patterns of the stencil mask; (b) projecting the electron beams passed through the mark patterns onto the surface of the sample through the reduction optical system and undeflected by the irradiation deflector to form images of the mark patterns on the surface of the sample, the images being in a positional relationship; (c) computing the positional relationship of the images; and (d) comparing the positional relationship of the images with the positional relationship of the mark patterns on the stencil mask, thereby to detect a reduction ratio of the images on the surface of the sample and a rotational angle of the images.