A theory of the L-mode confinement in tokamaks is developed based on the microscopic ballooning instability which is destabilized by the plasma transport below the critical pressure gradient against the ideal MHD instability. Destabilization by the current diffusivity and stabilization by the thermal transport and ion viscosity are analysed. The least stable mode determines the anomalous transport coefficients. The formula of the thermal transport coefficient is derived, which explains major experimental observations on L-mode confinement.