In a linear trinuclear system ABA, when the A-B interaction is antiferromagnetic, the ground state does not have the lowest spin if 2S_A > S_B + 1/2, which is so called irregular spin state. This may lead to characteristic magnetic behavior with a minimum in the _χMT versus T plot. Below the temperature of the minimum, the compound exhibits a ferromagnetic- like behavior. The irregular spin state provides an efficient way to synthesize a molecular system with a high-spin multiplicity in the ground state, without imposing ferromagnetic interaction between nearest- neighbor ions. This review summarizes the main idea of the irregular spin state and the latest progress in this field.