Necessary conditions of rise and development of decay instability of low-frequency kinetic Alfven waves have been obtained in the loop structures before a flare in solar active regions. Expression for the nonlinear growth rate of decay instability of initial kinetic Alfven waves (KAW) for secondary wave (KAW) and kinetic ion-acoustic wave (KIAW) has been derived on the basis of transcendental dispersion relation which has been obtained with the help of the synchronism conditions and additional conservation laws. Boundary values of the reduced amplitude of initial KAW in preflare atmosphere of active region (AR) have been obtained. It has been proposed in calculations that waves in the area under investigation appear as result of instability development. This last one takes place due to existence of large-scale weak electric (subdreicer) field and drift motions of plasma densety and temperature. It has been shown that for some specific kind of semiempirical model of solar atmosphere the KAWs and KIAWs can be generated during linear stage of instability development. At the same time the process of wave generation can take place in plasma with pure coulomb conductivity as well as in plasma with small-scale Bernstein turbulence. For the start up of instability the relatively low values of plasma nonisothermality are needed as well as very small threshold values of the reduced amplitude of subdreicer electric field. Appearance of nondamped kinetic waves with small amplitude in the area under investigation has been proved.