The lasing-induced change in the differential resistance is measured for various types of stripe geometry laser, and a model is given that explains the lasing induced change quantitatively to some extent. The amount of the lasing-induced reduction in the differential resistance is mainly determined by a parameter that limits the carrier supply into the active layer. The exsistence of a non-lasing region at the edge of the stripe and lateral carrier diffusion also decrease the amount of the reduction. In narrow-stripe lasers the amount of the reduction is small due to lateral carrier diffusion. From the moderate change in the differential resistance near the threshold, the spontaneous emission factor representing the contribution of the spontaneous emission to the lasing mode is estimated for various types of stripe laser.