An indoor unit of a refrigeration apparatus comprising: a drain pan that has four wall surfaces including a first wall surface and has a quadrangle shape in a plan view; a heat exchanger that is installed above the drain pan and through which a combustible refrigerant having a larger specific gravity than air flows; a fan that generates an air flow to the heat exchanger; a gas sensor that detects a refrigerant leakage; and a casing accommodating the drain pan, the heat exchanger, the fan, and the gas sensor, wherein the casing includes a plurality of side plates (23, 24, 26, 27) constituting side surfaces of an outer contour, a partition plate that divides an internal space surrounded by the plurality of side plates into a first chamber in which the drain pan is installed and a second chamber in which the fan is installed, and a blow-out port provided in the first side plate as one of the plurality of side plates, the first side plate faces the first wall surface of the drain pan, the wall surfaces of the drain pan other than the first wall surface are arranged along the side plates (24, 26) or the partition plate, characterized in that an installation position of the gas sensor is above the drain pan, and a height H from an upper end of the drain pan to the gas sensor satisfies a relational expression represented by L · W C 1 · H 1 / Q + C 2 · H / Q − C 3 · L · H ∧ 3 / 2 ≤ 90, where constant C1: 0.0067, constant C2: 0.01172, constant C3: 0.000153, L [m]: a length of the first wall surface of the drain pan, W [m]: a length of the wall surface of the drain pan intersecting with the first wall surface, H1 [m]: a depth of the drain pan, and Q [m ^ 3/s]: a refrigerant leakage flow rate. a drain pan that has four wall surfaces including a first wall surface and has a quadrangle shape in a plan view; a heat exchanger that is installed above the drain pan and through which a combustible refrigerant having a larger specific gravity than air flows; a fan that generates an air flow to the heat exchanger; a gas sensor that detects a refrigerant leakage; and a casing accommodating the drain pan, the heat exchanger, the fan, and the gas sensor, wherein the casing includes a plurality of side plates (23, 24, 26, 27) constituting side surfaces of an outer contour, a partition plate that divides an internal space surrounded by the plurality of side plates into a first chamber in which the drain pan is installed and a second chamber in which the fan is installed, and a blow-out port provided in the first side plate as one of the plurality of side plates, the first side plate faces the first wall surface of the drain pan, the wall surfaces of the drain pan other than the first wall surface are arranged along the side plates (24, 26) or the partition plate, characterized in that an installation position of the gas sensor is above the drain pan, and a height H from an upper end of the drain pan to the gas sensor satisfies a relational expression represented by L · W C 1 · H 1 / Q + C 2 · H / Q − C 3 · L · H ∧ 3 / 2 ≤ 90, where constant C1: 0.0067, constant C2: 0.01172, constant C3: 0.000153, L [m]: a length of the first wall surface of the drain pan, W [m]: a length of the wall surface of the drain pan intersecting with the first wall surface, H1 [m]: a depth of the drain pan, and Q [m ^ 3/s]: a refrigerant leakage flow rate. constant C1: 0.0067, constant C2: 0.01172, constant C3: 0.000153, L [m]: a length of the first wall surface of the drain pan, W [m]: a length of the wall surface of the drain pan intersecting with the first wall surface, H1 [m]: a depth of the drain pan, and Q [m ^ 3/s]: a refrigerant leakage flow rate.