The decay of excited states in $^{255}\mathrm{No}$ was investigated by applying the evaporation-residue--conversion-electron correlation technique. Two new isomeric states were observed in $^{255}\mathrm{No}$ together with the previously known one. Excitation energies of the isomeric states were estimated based on the energies of conversion electrons and $\ensuremath{\gamma}$ rays from correlation chains. These results were in accord with theoretical calculations based on the mean-field models. A tentative decay scheme of isomeric states in $^{255}\mathrm{No}$ is proposed, and their Nilsson configurations are discussed. The energy decrease of the $11/{2}^{\ensuremath{-}}$[725] Nilsson level for heavy $N=153$ isotones as a function of increasing proton number is confirmed.