To provide a cylinder gas temperature estimation device for an internal combustion engine accurately estimating cylinder gas temperature when a cavity is formed in the top of a piston.

When estimating the cylinder gas temperature, it is assumed that all cylinder gases exist outside of the cavity if a crank angle is out of a range of CA1 and CA2 including a TDC (Top Dead Center), and the amount of heat loss of the cylinder gas is calculated based on "the amount of heat transmitted between the cylinder gas and a cylinder sidewall" calculated using a heat transfer coefficient h1 based on a swirl flow speed V1 with respect to the cylinder sidewall (Step 1018). It is assumed that all cylinder gases exist in the cavity if the crank angle is within the range of CA1 and CA2, and the amount of heat loss of the cylinder gas is calculated based on "the amount of heat transmitted between the cylinder gas and the cavity sidewall" calculated using a heat transfer coefficient h2 (>h1) based on a swirl flow speed V2 (>V1) increased due to spinning up with respect to the cavity sidewall (Step 1036).