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 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 (R1) in which the drain pan is installed and a second chamber (R2) 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 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 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 (R1) in which the drain pan is installed and a second chamber (R2) 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 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. <EOS>