This paper introduces an analog-to-stochastic converter using a magnetic-tunnel junction (MTJ) device for stochastic computation. Stochastic computation has recently been exploited for area-efficient hardware implementation, such as low-density parity-check (LDPC) decoders and image processors. However, power-and-area hungry analog-to-digital and digital-to-stochastic converters are required for the analog to stochastic signal conversion. The MTJ devices exhibit probabilistic switching behaviour between two resistance states. Exploiting the probabilistic behaviour, analog signals can be directly converted to stochastic signals to mitigate the signal-conversion overhead. The analog-to-stochastic signal conversion is mathematically described and the conversion circuit is designed based on a transistor/MTJ hybrid structure. The conversion characteristic is evaluated using device and circuit parameters that determines proper parameters for designing the analog-to-stochastic con-verter.