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An analysis is presented of the optical polarimetric and multicolourphotometric ($BVRJ$) behaviour of the blazar PKS 2155$-$304 during an outburstin 2010. This flare develops over roughly 117 days, with a flux doubling time$\tau \sim 11$ days that increases from blue to red wavelengths. Thepolarization angle is initially aligned with the jet axis but rotates byroughly $90^\circ$ as the flare grows. Two distinct states are evident at lowand high fluxes. Below 18 mJy, the polarization angle takes on a wide range ofvalues, without any clear relation to the flux. In contrast, there is apositive correlation between the polarization angle and flux above 18 mJy. Thepolarization degree does not display a clear correlation with the flux. We findthat the photopolarimetric behaviour for the high flux state can be attributedto a variable component with a steady power-law spectral energy distributionand high optical polarization degree (13.3%). These properties are interpretedwithin the shock-in-jet model, which shows that the observed variability can beexplained by a shock that is seen nearly edge-on. Some parameters derived forthe relativistic jet within the shock-in-jet model are: $B=0.06$ G for themagnetic field, $\delta=22.3$ for the Doppler factor and $\Phi=2.6^\circ$ forthe viewing angle.

The optical polarization of the blazar PKS 2155$-$304 during an optical flare in 2010