A model is suggested to explain the variability of the optical structure and the integral brightness of cometary nebulae (CN) occurring at timescales of several years and tens of years (Gyulbudaghian et al. 1977, Cohen et al. 1977, 1981, Magakyan 1981, Gyulbudaghian 1982). A CN is assumed to be a reflection nebula; it is a wall of a conical cavity in the circumstellar gas-and-dust torus illuminated by the central star (Cohen 1974). I explain the CN's variability by the presence of small tilted circumstellar disc of gas-and-dust, located inside the internal channel of the large circumstellar torus (see Figure 1). A similar model was put forward by Ward-Thompson et al. (1985) to account for a tilt angle of about 30° between the direction of short optical jets (stellar wind, channelled by the small disc) and the large-scale bipolar outflow (focused by the large torus) in the CN NGC 6729 associated with the star R CrA. Tilt angles of about 30° between optical and radio structures exist in CN NGC 2261 (Cantó et al. 1981) and GM 1-29 (Levreault 1984).