The compound $({\mathrm{CH}}_{3}{)}_{2}{\mathrm{CHNH}}_{3}{\mathrm{CuCl}}_{3},$ which consists of ferromagnetic-dominant ferromagnetic and antiferromagnetic alternating Heisenberg chains with $S=1/2,$ is regarded as a Haldane system with $S=1$ at low temperatures because a pair of ferromagnetically coupled spins behaves as $S=1.$ There should therefore be a crossover of the spin state from $S=1/2$ to $S=1$ when the temperature is decreased. With the expectation that the crossover causes a drastic change in spin dynamics, electron paramagnetic resonance (EPR) experiments were performed at 24 GHz on single crystals of this compound over the region of 1.4--295 K. The EPR spectra observed below 10 K were found to show characteristics clearly distinct from those above 10 K. That is, a single absorption line observed above 10 K was found to split into two lines below 10 K, and an additional weak line appeared at a position corresponding to half of the averaged resonance fields of the two lines. The resonance fields of the two lines vary with the direction of the external field H; their angular dependence is $a\ifmmode\pm\else\textpm\fi{}b(1\ensuremath{-}3{\mathrm{cos}}^{2}\ensuremath{\theta}),$ where a and b are constants, and $\ensuremath{\theta}$ is the angle between the direction of H and one of the normals of the orthogonal crystal surfaces. The resonance field of the weak line that appeared at the half-field position was almost constant with respect to the direction of H. These experimental results observed below 10 K are explained when one considers that the $S=1$ state caused by pairs of ferromagnetically coupled two spins supersedes the $S=1/2$ state of the individual spins. Then the dipole--dipole interaction $({\mathcal{H}}_{\mathrm{DD}}^{\ensuremath{'}})$ and the anisotropic exchange interaction $({\mathcal{H}}_{\mathrm{AE}}^{\ensuremath{'}})$ between ferromagnetically coupled two spins act as a fictitious single ion anisotropy, and remove the threefold degeneracy of the triplet state of $S=1,$ i.e., ${E}_{{S,S}_{z}}{=E}_{1,1}{,E}_{1,0},$ and ${E}_{1,\ensuremath{-}1}.$ As a result, the $\ensuremath{\Delta}{S}_{z}=\ifmmode\pm\else\textpm\fi{}1$ transitions, i.e., the transitions between ${E}_{1,\ensuremath{-}1}$ and ${E}_{1,0},$ and between ${E}_{1,0}$ and ${E}_{1,1}$ bring about two absorption lines. That is why the two lines appear below 10 K. The weak half-field line is due to the $\ensuremath{\Delta}{S}_{z}=\ifmmode\pm\else\textpm\fi{}2$ transition, which is also caused by ${\mathcal{H}}_{\mathrm{DD}}^{\ensuremath{'}}$ and ${\mathcal{H}}_{\mathrm{AE}}^{\ensuremath{'}}$ between ferromagnetically coupled two spins because nondiagonal elements between $|1,1〉$ and $|1,\ensuremath{-}1〉$ are not 0.