A compressor which can be easily assembled and can prevent damage of seal members is presented. Its assembling method includes of (a) a step of feeding a front casing having a first inner circumference, a second inner circumference, an opening, a bottom, and a side wall, compression units having a first fitting portion and a first seal groove, and a rear cover having a second fitting portion and a second seal groove, (b) a step of installing a first seal member in the fist seal groove, (c) a step of installing s second seal member in the second seal groove, (e) a step of installing the fixed scroll on the front casing, while fitting the first fitting portion to the first inner circumference, and (f) a step of installing the rear cover to the front casing so as to cover the opening, while fitting the second fitting portion to the second inner circumference. The first fitting gap between the first fitting portion and the first inner circumference is larger than the second fitting gap of the second fitting portion and the second inner circumference. The steps (e) and (f) are the process of inserting the compression unit and the rear cover mutually coupled and mounting the first seal member and the second seal member into the front casing from the opening.