It has been considered that an amount of calcium elevation determines the synaptic plasticity. However, it has been pointed out that simple application of this principle cannot reproduce the timing dependence of spike-timing-dependent plasticity (STDP) [1, 2]. A few mechanisms were previously proposed to solve the problem [3, 4]. These are possible mechanisms, but it still remains unclear what mechanism is true for actual STDP. Here, we present two other mechanisms to solve the problem. One is a fixed threshold model with a finite time window for the calcium elevation, and the other one is a mechanism using dynamically sliding threshold determined as the linear summation of calcium elevations induced by single pre-synaptic and post-synaptic spikes.